Make NotInSchema field private

[stepancheg]

NotInSchema error can have a cheap enum_name: &'static str field to make error message more helpful, because when we parse files, we usually don't provide detailed information where error comes from. So storing such information would help debugging it.

But that would be a breaking change. So let's break it now.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 47 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.98%. Comparing base (ab342b3) to head (566bc07).
⚠️ Report is 351 commits behind head on master.

Files with missing lines	Patch %	Lines
capnp/src/schema_capnp.rs	0.00%	19 Missing ⚠️
capnp-rpc/src/rpc_capnp.rs	0.00%	17 Missing ⚠️
capnp/src/lib.rs	0.00%	7 Missing ⚠️
capnpc/src/codegen.rs	0.00%	2 Missing ⚠️
capnp-rpc/src/rpc.rs	0.00%	1 Missing ⚠️
capnp-rpc/src/rpc_twoparty_capnp.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #634      +/-   ##
==========================================
- Coverage   51.64%   50.98%   -0.66%     
==========================================
  Files          69       71       +2     
  Lines       33735    32506    -1229     
==========================================
- Hits        17422    16574     -848     
+ Misses      16313    15932     -381     

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