Allow other applications to hook into showerror

[Seelengrab]

Closes #68.

This change allows other applications to customize showerror(::IO, ::JSONRPCError) depending on their specific needs and chosen meanings for their errors. It also introduces the fixed error codes as constants, to allow them to be transparently referenced in other places of the codebase.

Regarding serverErrorStart and serverErrorEnd - I don't know exactly how they're used in the VSCode extension/LS.jl (are the messages themselves matched anywhere?), so these could be added explicitly until those dependents are explicitly adding them to RPCErrorStrings. I've added them for now to make tests pass, but having them implicit seems cleaner to me.

For every PR, please check the following:

• End-user documentation check. If this PR requires end-user documentation in the Julia VS Code extension docs, please add that at https://github.com/julia-vscode/docs.
• Changelog mention. If this PR should be mentioned in the CHANGELOG for the Julia VS Code extension, please open a PR against https://github.com/julia-vscode/julia-vscode/blob/master/CHANGELOG.md with those changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.65%. Comparing base (5adc04b) to head (962d276).
Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   91.46%   89.65%   -1.81%     
==========================================
  Files           4        4              
  Lines         246      232      -14     
==========================================
- Hits          225      208      -17     
- Misses         21       24       +3     

Flag	Coverage Δ
unittests	89.65% <100.00%> (-1.81%)	⬇️

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[Seelengrab]

Not quite sure what the coverage diff is about - it seems to be inconsistent in terms of what it compares? Or I'm clicking on the wrong thing, not sure.

[Seelengrab]

The original intention here was to keep the LSP specific ones around until the time that LanguageServer.jl can/wants to integrate the functionality, but since that would require compat bumping sooner or later anyway, might as well just remove them now and do a lockstep release with appropriate compat upper bounds and/or upper bounding previous versions in the registry.

The question is just - is this a technically breaking release, since it changes the error message of LanguageServer.jl from e.g. ContentModified to Unknown if JSONRPC.jl is held back? A version of LanguageServer.jl making use of this functionality definitely needs lower bound on the new version (presumably 1.4, since this is a new feature).

[pfitzseb]

The question is just - is this a technically breaking release, since it changes the error message of LanguageServer.jl from e.g. ContentModified to Unknown if JSONRPC.jl is held back?

Mh, not sure. The error type doesn't change after all. I don't mind tagging a 2.0 for correctness either way, but imho that wouldn't really be necessary.

[Seelengrab]

Mh, not sure. The error type doesn't change after all. I don't mind tagging a 2.0 for correctness either way, but imho that wouldn't really be necessary.

Yeah, I don't mind either way :) Whichever is more convenient to have this percolate through the dependencies without causing too much disruption.

Is there some NEWS file or release notes specific to JSONRPC.jl that this should be mentioned in as a new feature?