Improve error message on exception raised by get_default

This commit adds a missing message from the InvalidConfiguration
exception raised by the get_default method in the credentials classes.
Without an explanatory message it is very difficult for users to
identify the cause of the configuration error.

Change-Id: I709500fd32bdc86766de4f72e2032591c2f7aaa3
Merge "Improve error message on exception raised by get_default"