proto: Add UserCommitFilesError

Add a new UserCommitFilesError message type that will be used to add structured errors to the UserCommitFiles RPC. This necessitates creating a new IndexError error type to enumerates the type of index operations that may fail.

We limit IndexError to a single path despite a potentially large number of files being modified because we end the RPC on the first failure, so any subsequent paths that would have failed are not encountered. path is sent as bytes in the proto because file paths are not guaranteed to be valid UTF-8 by many common filesytems, which is the required encoding for protobuf strings.

Related to #4472 (closed)