Put fault message in the correct field

Currently when adding an instance fault, the message field looks
for kwargs['value'], which many exceptions do not have.  This
results in the message field being set to the name of the
exception class, which in some cases is not useful (see the bug
report for an example).

This change puts the exception message in the message field, and
removes it from the details field since there's no need to put it
in both places.  That leaves the details field for actual details,
like stack traces.

Fixes bug 1203880

Change-Id: Ic92a9bd9cb2e139ce7a9f3f1299b7a6ccae82dd1
Merge "Put fault message in the correct field"