Avoid shadowing Exception 'message' attribute

Exception.message is deprecated in Python 3, and it's confusing
that we shadow it right now, so change class attributes named
message to something else.

For reference, see commit 7bfd4439fe48e861675e3930486b55f8badd072e

Change-Id: I913a8a5d025cc392ef2003951763cf55629fd8d2
Merge "Avoid shadowing Exception 'message' attribute"