A small step to closing "gracefully handle md5 mismatch"[0]

I learned straight `raise Exception()` is bad practice, because it can hide
exceptions _other_ than what you were after. Aaron Hall is worth paying
attention to![1]

 [0] http://trac.osgeo.org/osgeo4w/ticket/210
 [1] http://stackoverflow.com/a/24065533/14420