Alias merger fix

[MHendricks]

Checklist

• [x] I have read the CONTRIBUTING.md document
• [x] I formatted my changes with black
• [x] I linted my changes with flake8
• [x] I have added documentation regarding my changes where necessary
• [x] Any pre-existing tests continue to pass
• [x] Additional tests were made covering my changes

Types of Changes

• [x] Bugfix (change that fixes an issue)
• [ ] New Feature (change that adds functionality)
• [ ] Documentation Update (if none of the other choices apply)

Proposed Changes

If you try to use {PATH!e} in an alias environment variable, it raises an KeyError exception. This makes it properly handle that.