Use FailedPrecondition in Trillian Test. (#1086)
Use FailedPrecondition in Trillian Test. (#1086)

Use the same error code as the Trilian code base does for uninitalized
trees while waiting for the tree to be initialized.

This follows change google/trillian#1333
gdbelvin/keytransparency,gdbelvin/keytransparency,google/keytransparency,google/keytransparency