Change module root find function in CompatibilityRule

[rjmholt]

PR Summary

After @bergmeister's comments in @JamesWTruher's PR, I've made some small changes to the function that finds the PSScriptAnalyzer module root, the main one being that it will explicitly throw a FileNotFoundException when it gives up.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[rjmholt]

Will let someone else merge