Fix a bug that `ra:restart_server/2` could raise an exception

[sile]

Proposed Changes

This PR fixes a bug that ra:restart_server/2 could raise a case_clause exception as the function doesn't handle {badrpc, _} (a possible return value of ra_server_sup_sup:restart_server()).

I ran into this bug when I tested how ra works if disk failure happens (as an example of the exception, see the stacktrace below).

{{case_clause,
  {badrpc,
   {'EXIT',
    {badarg,
     [{dets,lookup,
       [ra_directory_reverse, foo],
       [{file,"dets.erl"},{line,1269}]},
      {ra_directory,uid_of,2,
       [{file,
         "/foo/_build/default/lib/ra/src/ra_directory.erl"},
        {line,166}]},
      {ra_server_sup_sup,recover_config,2,
       [{file,
         "/foo/_build/default/lib/ra/src/ra_server_sup_sup.erl"},
        {line,203}]},
      {ra_server_sup_sup,restart_server_rpc,3,
       [{file,
         "/foo/_build/default/lib/ra/src/ra_server_sup_sup.erl"},
        {line,86}]}]}}}},
 [{ra,restart_server,2,
   [{file,
     "/foo/_build/default/lib/ra/src/ra.erl"},
    {line,174}]}, ...

I could not make a reproducing code or a test case that cover this case as it's too rare and hard.

Types of Changes

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• [x] Bug fix (non-breaking change which fixes issue #NNNN)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation (correction or otherwise)
• [ ] Cosmetics (whitespace, appearance)

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• [x] I have read the CONTRIBUTING.md document
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• [x] All tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have added necessary documentation (if appropriate)
• [x] Any dependent changes have been merged and published in related repositories

Further Comments

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[kjnilsson]

Thanks!