Adding more descriptive error message

Keystone Trusts API sends non descriptive error message for requests with
invalid trust id.
The error message only contains invalid_trust_id  instead of printing:
"Could not find trust: invalid_trust_id"
Setting kwarg "trust_id" in the exception.

Closes-Bug: #1305272
Change-Id: I35cab35ab90cf66ce7124dea142013bec9eefda9