Handle exceptions from Roslyn Completion API

[waf]

Workaround for #331. We get the following errors from the roslyn completion libs:

InvalidOperationException: This program location is thought to be unreachable. File='/_/src/Features/CSharp/Portable/Completion/CompletionProviders/PropertySubPatternCompletionProvider.cs' Line=116

If we get any roslyn library exceptions during autocompletion, it's better to just not have any completions than crash the entire repl.

[codecov]

Codecov Report

Attention: Patch coverage is 64.28571% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 77.6%. Comparing base (9eff460) to head (55ce52a).
Report is 3 commits behind head on main.

Files	Patch %	Lines
...arpRepl.Services/Completion/AutoCompleteService.cs	64.2%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #334     +/-   ##
=======================================
- Coverage   77.6%   77.6%   -0.1%     
=======================================
  Files         83      83             
  Lines       5597    5600      +3     
  Branches     734     730      -4     
=======================================
+ Hits        4346    4347      +1     
- Misses       984     986      +2     
  Partials     267     267             

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