Annotating the exception thrown when encountering a bad line in a datacard file

Added code to annotate the exception thrown when encountering 
a bad line from the datacards file with the line number and file name if known.

Most of the diff shown below is actually a necessary increase in
indendation of the code. To better visualize the differences (without the whitespace differences), do on a command line:

 git diff -w e5ec20b..53fa568