Crosswalk warnings and errors for bad data

[gregjan]

We need to show the user more feedback from crosswalk runs. Some examples:

• Warn if a particular XML element does not get a text value.
• Warn if an input value doesn't match any of the patterns in their date maker.
• Warn if the user hasn't mapped something and there is no default value.
• Error if a output MODS record is completely empty.
• Error if there is a matcher but nothing is matched

These all seem like errors or warnings tied to the crosswalk definition. There are probably many more.

It might be nice if the user could also require certain output elements, such that if the crosswalk didn't output that element, it would output an Error.