Fix error type to match `sendTreeEntries` interface
The change in 64549d6f
and especially turning on the feature flag to use the new
sendTreeEntriesUnified
broke the GetTreeEntries
RPC interface with
respect to error messages (which I'd consider part of the interface,
even though not explicitly declared).
This change set aligns the error message on a git.ErrReferenceNotFound
in the sendTreeEntriesUnified
to match the one in sendTreeEntries
.
See gitlab#443261 (closed) for more context.