Add safe_reraise() + warnings only to Connection._autorollback

Add safe_reraise() + warnings only to Connection._autorollback

Added an exception handler that will warn for the "cause" exception on
Py2K when the "autorollback" feature of :class:`.Connection` itself
raises an exception. In Py3K, the two exceptions are naturally reported
by the interpreter as one occurring during the handling of the other.
This is continuing with the series of changes for rollback failure
handling that were last visited as part of :ticket:`2696` in 1.0.12.

Change-Id: I600ba455a14ebaea27c6189889181f97c632f179
Fixes: #3946