Output task not found messages to *err*

Throws an ex-info rather than calling abort to avoid the "Leiningen has
encountered a critical error and will now exit." message.

Adds a check for a :suppress-msg key before printing the exception message in
leiningen.core.main/main.
Merge pull request #1024 from hugoduncan/feature/task-not-found-to-stderr

Output task not found messages to *err*