Add error reporting

[crittermike]

Some error reporting would be nice. For example, Flatfish doesn't alert the user when it tries to migrate a page that doesn't exist (it just adds a row with empty content columns) or when it tries to migrate a nonexistent region from a page (replace .region-content-inner with .does-not-exist in article.csv for example).

IMO it would be nice if it reported errors as they happened (so you can stop processing as early as possible to fix them) and then also as a little summary at the end.

Any thoughts on this? This obviously is just a nice-to-have.

[tloudon]

Let's flesh this out a bit next week. There might be some issues changing things with the way the code flows, but I'd like to spend a little time going over use cases so we get things right.