zebrad instead of zcashd in error messages

[LarryRuane]

Closes #488. Indicate the correct backend process name in error messages. Some error messages occur before we know if our backend is zcashd or zebrad (for example, when we're attempting the first RPC), so those will mention both as possibilities. A follow-on PR could mention the backend name in additional error messages (this PR corrects the only two, only one of which is a gRPC, GetTreeState(), the other in the block ingestor). I did test this PR with an instance of zebrad.