Bug 923791 - [Messages] Fix SMS failure mocking on desktop r=rwaldron

https://bugzilla.mozilla.org/show_bug.cgi?id=923791

Reflect current `SmsRequest` API using `event.target.error`.

Rename `window.SMSDebugFail` to `window.MessagesDebugError` to reflect
how it's a string representing the error name.

Signed-off-by: Rick Waldron <waldron.rick@gmail.com>