Add capability to register and unregister failure handlers

[j-concepcion]

Failure handlers are triggered when the email fails to send and an exception is raised.

Tested against:

• 1.8.7-p375, "mime-types-1.16" appraisal (newer specified versions of mime-types seem to require >= ruby 1.9.3)
• 1.9.3-p551 (tested all appraisals specified)
• 2.2.2 (tested all appraisals specified)

[ramontayag]

I worked on this with @j-concepcion. Is there anything we can do to get this merged? Do you want us to test anything else?

[ramontayag]

Just updated this to 1) get it mergeable again, 2) follow the format of changelog

[jeremy]

Cool idea 👍

[ramontayag]

Thanks! It looks like it's failing for some other reason.