http/500 when package is not part of release

[al4]

Should be a useful error message rather than 500

[2016-08-09 16:16:31 +0000] [2642] [ERROR] Error handling request /releases/9a063e34-03f5-4b9a-991a-4e5278468a94/packages/bfb0-4eec-860e-bd211c62ccc9/stop
Traceback (most recent call last):
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 135, in handle
    self.handle_request(listener, req, client, addr)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 176, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 2000, in __call__
    return self.wsgi_app(environ, start_response)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1991, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1567, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1988, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1641, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1544, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1639, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/flask/app.py", line 1625, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/orlo/user_auth.py", line 42, in wrapped
    return func(*args, **kwargs)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/orlo/route_releases.py", line 249, in post_packages_stop
    package = fetch_package(release_id, package_id)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/orlo/util.py", line 95, in fetch_package
    package = pq.first()
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2659, in first
    ret = list(self[0:1])
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2457, in __getitem__
    return list(res)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2761, in __iter__
    return self._execute_and_instances(context)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2776, in _execute_and_instances
    result = conn.execute(querycontext.statement, self._params)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
    return meth(self, multiparams, params)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
    return connection._execute_clauseelement(self, multiparams, params)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
    compiled_sql, distilled_params
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1078, in _execute_context
    None, None)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1341, in _handle_dbapi_exception
    exc_info
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/util/compat.py", line 202, in raise_from_cause
    reraise(type(exception), exception, tb=exc_tb, cause=cause)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/base.py", line 1073, in _execute_context
    context = constructor(dialect, self, conn, *args)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 600, in _init_compiled
    for key in compiled_params
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/engine/default.py", line 600, in <genexpr>
    for key in compiled_params
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy/sql/type_api.py", line 978, in process
    return impl_processor(process_param(value, dialect))
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy_utils/types/uuid.py", line 64, in process_bind_param
    value = self._coerce(value)
  File "/usr/share/python/orlo/local/lib/python2.7/site-packages/sqlalchemy_utils/types/uuid.py", line 55, in _coerce
    value = uuid.UUID(bytes=value)
  File "/usr/lib/python2.7/uuid.py", line 144, in __init__
    raise ValueError('bytes is not a 16-char string')
StatementError: (exceptions.ValueError) bytes is not a 16-char string [SQL: u'SELECT package.id AS package_id, package.name AS package_name, package.stime AS package_stime, package.ftime AS package_ftime, package.duration AS package_duration, package.status AS package_status, package.version AS package_version, package.diff_url AS package_diff_url, package.rollback AS package_rollback, package.release_id AS package_release_id \nFROM package \nWHERE package.id = %(id_1)s \n LIMIT %(param_1)s'] [parameters: [immutabledict({})]]