(PUP-2918) Clarify missing Modulefile error

Prior to this commit, when building a module without a Modulefile or metadata.json,
the builder would blow up with a cryptic 'Unable to find module root at DIRECTORY'
It's not entirely clear what that means to an end user.
This commit clarifies that metadata.json or a Modulefile are what need to be found
at the module root. Also removes some duplicated logic for checking module root.
Merge pull request #2857 from highb/fix/master/pup-2918_no_modulefile_error_message

(PUP-2918) Clarify missing Modulefile error