Remove all bare except clauses

While investigating aws/aws-cli#628 I noticed that I'd still
occasionally get tracebacks, particularly a DataNotFoundError
from botocore's base.py module.  After further investigation,
this was because the bare except clauses were catching the
KeyboardInterrupts and swallowing them.  This resulted in a
traceback somewhere downstream.

The specific error I hit from aws/aws-cli#628 was from base.py,
but I did a quick search through the code and for the time being
at least replaced the bare excepts with except Exception.  There's
potential to switch some of these to more specific exceptions later.