webhooks/mailgun: mark temporary failures as DEFERRED

[costela]

Also remap "generic" failure reason to RejectReason.OTHER, instead of
BOUNCE.

fixes #130

[costela]

Please note the minor style change when setting event_type for the first time. The .get() style (as opposed to try/except) was already being used for reject_reason and makes the code IMHO slightly more homogenous. Let me know if the try/except style is preferred.

[medmunds]

Looks great. Thanks for this.