Improve error handling

[floehopper]

Description of the change

This improves the error handling - giving better visibility of things that might have gone wrong. In particular:

• It now displays any error message returned in the response from Rollbar when the error code is not zero.
• It now displays a message if the deploy_id cannot be extracted from the response from Rollbar; whereas previously it just silently failed.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Related issues

N/A

Checklists

Development

• There don't appear to be any lint rules, so I haven't done anything about that.
• There don't appear to be any tests, so I haven't done anything about that.

Code review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• "Ready for review" label attached to the PR and reviewers mentioned in a comment
• Changes have been reviewed by at least one other engineer
• Issue from task tracker has a link to this pull request

[introkun]

lgtm