Add key info for RefBackendError for easier debugging

[uberspot]

Related to issue #430
Slightly hard to generalize and add more information (like current filename processed for example)
because references are set/read in multiple places in cli.py as well in various use cases.
It might be feasible to add nicer exception messages there too with the same approach.

New message looks like this:

no backend for ref type: gpsafasg
  for key ?{gpsafasg:targets/minikube-mysql/mysql/password||randomstr|base64}
Compile error: failed to compile target: minikube-mysql

Which is slightly more helpful but not perfect.