Disallow invalid characters in repo name from GH

[ericc572]

Solving https://github.com/codetriage/codetriage/issues/1145:

Disallow invalid characters such as ?= in a username/name parameter. Throw error safely and exit.

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.