Only print and exit on errors. Warnings of the type
'PackageDistSuspicious' are not very useful.

The following show up in the build logs currently:
  * The 'license' field is missing.
  * No 'maintainer' field
  * No 'category' field
  * 'ghc-options: -O2' is rarely needed. Check that it is giving a real
    benefit and not just imposing longer compile times on your users.

Differential Revision: https://phabricator.haskell.org/D944