Make better error message when using wrong path in quickmodel inputs file.

[sonyahanson]

Currently it says this:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-17-e405fa54a26a> in <module>()
----> 1 [complex_fluorescence, ligand_fluorescence] = parser.get_data_using_inputs(inputs)

/Users/hansons/anaconda2/lib/python2.7/site-packages/assaytools-0.2.0-py2.7.egg/assaytools/parser.pyc in get_data_using_inputs(inputs)
     69         # data collection, e.g. if we wanted a lot of types of reads
     70         # (there is a limited number of type of read on the infinite per script).
---> 71         data = platereader.read_icontrol_xml(inputs['file_set']['%s'%protein][0])
     72         try:
     73             data.update(platereader.read_icontrol_xml(inputs['file_set']['%s'%protein][1]))

IndexError: list index out of range

Which is super opaque.