Correctly handle MissingRequirement from packages

Bug introduced by 447841f6
Removes unused imports
Revert "Eliminate GUI error messages"

This reverts commit 25ec19b82ada645d6d96457a4f8f9b9695b39a7e.
Removes bogus print statements
Changes missing startup file from CRITICAL to INFO
Merge pull request #2 from UV-CDAT/uvcdat-520-fix-packages

Uvcdat 520 fix packages