refactor: Added msgerrors package and refactored code for safer error handling

[KrishanBhalla]

Info

References #21
Attempts to limit service failure if a receivers are invalid.

Proposed changes

1. Add msgerrors package
2. Added unit tests for msgerrors
3. Collate errors within (notifier).Send, and return a new error after attempting to send to all recievers
4. Refactor of all relevant services to support this

Note

This is a proposed change that may require some iteration.
Question: msgerrors package name / function names - is there something clearer we should use instead?

Tested the Slack, Pushbullet, and PushbulletSMS services locally - all seems fine.

[nikoksr]

Closing this because of this comment.