#394 Fix bug for 1-line catalogs, and also remove try-except-raise pattern when not helpful

This is nominally a fix for a bug the Jim found where the InputCatalog didn't handle one-line catalogs correctly.  

However, more significantly, he pointed out that our somewhat common style of catching exceptions and then raising a new exception isn't actually helpful, since it hides the underlying error from the traceback that python produces.  So he removed some of these, and then I removed the rest from our python layer code.

Jim and I both think this is a better style.  But that question is probably more what this pull request is about.    The original bug-fix was pretty simple.  :)