pybind/mgr: improve behavior of ErrorResponse.wrap method

pybind/mgr: improve behavior of ErrorResponse.wrap method

The wrap classmethod is intended to turn any exception into something
that can produce a manager response. All exceptions inheriting from
ErrorResponseBase can do that, so instead of losing all that state,
have wrap return those exceptions inheriting from ErrorResponseBase
as-is.

Additionally, only change the sign of errno values that aren't already
negative.

Adds tests for these cases.

Signed-off-by: John Mulligan <jmulligan@redhat.com>