Stop swallowing handler errors when errors='raise'

[RazerM]

handle_error kept the flag check and the stacktrace print in one
try/except OSError. That except is there for the print, but it also
caught the exception handle_error re-raises for 'raise'. Most handler
failures are OSError, so most of them raised nothing.

The except predates the flag. In 2010 it wrapped the print alone, and
the raise landed inside it when the flag stack arrived two months
later. CPython scopes the same suppression to the print, see
python/cpython#50221.

[codecov]

Codecov Report

❌ Patch coverage is 98.30508% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 85.18%. Comparing base (80c60d7) to head (bc2c064).

Files with missing lines	Patch %	Lines
tests/test_flags.py	98.07%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #629      +/-   ##
==========================================
+ Coverage   85.04%   85.18%   +0.13%     
==========================================
  Files          45       45              
  Lines        5691     5743      +52     
  Branches      459      460       +1     
==========================================
+ Hits         4840     4892      +52     
  Misses        721      721              
  Partials      130      130              

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