svn2git does not error out with missing author in authors file

[lucianocheng]

When svn2git encounters a SVN commit author not in the authors file, it will print out an error but keep going in the migration. The error normally looks something like this:

Author: x not defined in authors.txt file

Even though the svn migration didn't complete successfully, svn2git will keep going and build the branches and tags, which buries the error.

I'd rather have svn2git error out than keep going on a incomplete migration, or have a flag to force this behavior.