Fix docstrings in enforce() and check() policy methods

Proper name of the  parameter is 'target', not 'object'.

Change-Id: Ia7a671c9aed644137e4c46452d8d8bf3bc3f2eee
Some v2 exceptions raise unicodeError

Use utils.exception_to_str to consistently cast the
excpetion to avoid the unicodeError stack trace.

Change-Id: I54fce91c73adca846ec99ed50a57b95d16428ec1
Closes-Bug: #1339775
Merge "Fix docstrings in enforce() and check() policy methods"
Merge "Some v2 exceptions raise unicodeError"