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[bot]]

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 :warning:

Additional details and impacted files

```diff @@ 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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