grpc-labview ought to make better use of C++ exceptions

[bkeryan]

The grpc-labview C++ codebase has a lot of duplicated error handling, and #436 adds more.

Instead of duplicating checks for failed casts, etc. in each exported function, the checks could be single-sourced:

• Create a C++ exception class for an exception with an error code.
• Change CastTo to throw an exception with the appropriate error code when it fails
• Add a TryCastTo method for use cases that require casting to null, and update callers to use it when appropriate.
• Add an exception dispatcher method like

  int32_t translateException() {
      try { 
          throw; 
      } catch (const MyExceptionWithErrorCode& e) {
          return e.code;
      } catch (const std::exception& e) {
          logException(e);
          return SOME_ERROR_CODE;
      }
  }

• Add a try/catch block to each exported function that uses the exception dispatcher:

  try {
      ...
  } catch (const std::exception&) {
      return translateException();
  }

  It is possible to wrap this in a macro, but not necessary, since most of the logic is single sourced in the exception dispatcher function.

The code already does some of this, but only for invalid enum values.

Originally posted by @bkeryan in #436 (comment)

AB#3117529

[bkeryan]

The feature toggle code would also benefit from being able to use C++ exceptions to report errors. Currently it ignores errors from
opening std::ifstream, GetModuleHandle, etc. It makes sense to ignore file-not-found when using the default feature toggle file path, but other errors ought to be reported to the caller.

[bkeryan]

#465 implements some of this:

• Create a C++ exception class for an exception with an error code.
• Change CastTo to throw an exception with the appropriate error code when it fails
• Add a TryCastTo method for use cases that require casting to null, and update callers to use it when appropriate.
• Add an exception dispatcher method
• Add a try/catch block to each exported function that uses the exception dispatcher: