StandardError -> Exception

StandardError doesn't exist in Python3, so we need to use the
slightly looser Exception
Fix some catching of exceptions
add stopgap warning
Merge remote-tracking branch 'origin/develop' into ticket/15620
Replace two more "except StandardError:"
Trac #15748: stopgap warning for power series equality

#9457 tracks a bug in power series equality.  This ticket gives a
warning for that bug.

URL: http://trac.sagemath.org/15748
Reported by: niles
Ticket author(s): Niles Johnson
Reviewer(s): Ralf Stephan
Trac #15620: StandardError is not Python 3 compatible

The simple solution is to replace all usage of `StandardError` with
`Exception`.

URL: http://trac.sagemath.org/15620
Reported by: ohanar
Ticket author(s): R. Andrew Ohana, Jeroen Demeyer
Reviewer(s): Jeroen Demeyer, R. Andrew Ohana