Create exam: show a better error message for syntax errors in the XML file

[jfisteus]

Right now, this is dumped to the output:

Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/jaf/otros/programas/eyegrade/eyegrade/create_exam.py", line 235, in <module>
    main()
  File "/home/jaf/otros/programas/eyegrade/eyegrade/create_exam.py", line 230, in main
    create_exam()
  File "/home/jaf/otros/programas/eyegrade/eyegrade/create_exam.py", line 130, in create_exam
    exam = utils.read_exam_questions(options.exam_filename)
  File "/home/jaf/otros/programas/eyegrade/eyegrade/utils.py", line 1047, in read_exam_questions
    dom_tree = xml.dom.minidom.parse(exam_filename)
  File "/usr/lib/python2.7/xml/dom/minidom.py", line 1918, in parse
    return expatbuilder.parse(file)
  File "/usr/lib/python2.7/xml/dom/expatbuilder.py", line 924, in parse
    result = builder.parseFile(fp)
  File "/usr/lib/python2.7/xml/dom/expatbuilder.py", line 207, in parseFile
    parser.Parse(buffer, 0)
xml.parsers.expat.ExpatError: not well-formed (invalid token): line 19, column 18