Issue a warning for missing footprint field

Created by: Ratfink

As pointed out in #234 (closed), there are plenty of cases where a symbol having one footprint filter and no default footprint is valid, and therefore this shouldn't be treated as an error by the script.

This commit changes that case to a warning, splits up the message into a warning and a warningExtra, and shortens the message overall by not explicitly saying how to fix the problem if it in fact is a problem.

Fixes #234 (closed).