Add error messaging to user when Kani crashes during coverage

[jaisnan]

Description of changes:

Adds error messaging to the user when there in an underlying crash from Kani during coverage highlighting.

Resolved issues:

Resolves #126

Testing:

• How is this change tested? Manually

• Is this a refactor change? No

Checklist

• Each commit message has a non-empty body, explaining why the change was made
• Methods or procedures are documented
• Regression or unit tests are included, or existing tests cover the modified code
• My PR is restricted to a single feature or bugfix

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.