Catch issues with bad MMSC URIs, add validation

Fixes #2202
// FREEBIE
Replace button text to cancel TS registration
in RegistrationActivity from "skip" to "cancel", when coming from the reminder

Closes #2219
Merge pull request #2230 from mcginty/manual-mms-pref-validation

Catch issues with bad MMSC URIs, add validation
merged  WhisperSystems/master