More ergonomic which

[stepancheg]

In which() return Result<Which<Reader>, Error> instead of Result<Which<Result<Reader, Error>>, NotInSchema>.

Extracting NotInSchema is tricker now, because it is now a variant of Error. Also it is not possible now have more detailed errors which variant is broken.

I think it's OK, because:

• in majority of cases people don't handle of every error in every enum
• being able to do exhaustive matching ergonomically is important, e.g.

match which? {
  Which::File(file) => { ... }
  Which::Directory(file) => { ... }
}

instead of

match which? {
  Which::File(Ok(file)) => { ... }
  Which::Directory(Ok(file)) => { ... }
  Which::File(Err(e)) | Which::Directory(Err(e)) => return Err(e),
}

• we should make errors more descriptive; I have a PR Make NotInSchema field private #634

[codecov]

Codecov Report

❌ Patch coverage is 23.28042% with 145 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.02%. Comparing base (ab342b3) to head (8e5d5a7).
⚠️ Report is 280 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp-rpc/src/rpc_capnp.rs	14.28%	60 Missing ⚠️
capnp/src/schema_capnp.rs	6.81%	41 Missing ⚠️
capnpc/src/codegen.rs	6.66%	28 Missing ⚠️
capnp-rpc/src/rpc.rs	61.76%	13 Missing ⚠️
capnpc/src/codegen_types.rs	0.00%	2 Missing ⚠️
capnp/src/dynamic_value.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #640      +/-   ##
==========================================
- Coverage   51.64%   51.02%   -0.62%     
==========================================
  Files          69       71       +2     
  Lines       33735    32406    -1329     
==========================================
- Hits        17422    16536     -886     
+ Misses      16313    15870     -443     

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