Rescue test settings for chat notification services

What does this MR do?

Rescue errors when testing chat notification services. Return error message to show user when testing chat notification services.

Are there points in the code the reviewer needs to double check?

Specs? Not sure if I should add them to the notification service spec (non specific) or the slack notification spec.

Why was this MR needed?

We blow up if we run into an exception when testing chat services.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?