commit: Use structured errors in `GetTreeEntries` rpc
In the GetTreeEntries rpc use structured errors for the following
scenarios:
- the provided path is invalid
- the provided revision is invalid
We add the usage of structured errors behind a featureflag
GetTreeEntriesStructuredErrors, which was introduced in a previous
commit. This allows for safe rollout. We also amend the tests and add
some new tests to cover more scenarios.
Closes #5036 (closed)