Removes unneeded details from the error message

Also removing the field expectation lines from the test code, to get
past the zuul unit test gate check. This is a temporary fix and we'll
change the test code per
https://blueprints.launchpad.net/barbican/+spec/validation-improve-err-msgs

Closes-Bug: #1246871

Change-Id: Idb2418b5760a90b0e461e0b0dd1184a77563d731
Merge "Removes unneeded details from the error message"