Misc. fixes

[yolken-segment]

Description

This change includes a number of fixes and improvements based on user feedback:

1. Throw an error during expansion if template uses a parameter that's not defined in the associated cluster config
2. Check for errors when walking directory paths; otherwise, if the path doesn't exist, this can cause a panic
3. Include more context in diff and apply errors generated during webhook runs
4. Fix bug in how --simple-output flag is handled in apply
5. Improve processing of errors when running an unstructured diff