Another Variant for detecting if a connection is closed

If there is a closed attribute on the connection and
it is true, return true. Implements a todo in the code
and helps in one specific disconnect case where it previously
did not match because the error message was "unknown error".
- The psycopg2 ``.closed`` accessor is now consulted when determining
if an exception is a "disconnect" error; ideally, this should remove
the need for any other inspection of the exception message to detect
disconnect, however we will leave those existing messages in place
as a fallback.   This should be able to handle newer cases like
"SSL EOF" conditions.  Pull request courtesy Dirk Mueller.
fixes #3021
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