Homogeneous treatment of parsing errors, with test coverage

[Eduardo Rodrigues]

Following up from #141 (comment):

The read_lhe type of functions and others, which parse the XML, do not all catch parsing errors.

1. We should discuss if parsing errors should raise errors or be dealt with smoothly, as now, with print statements. Any modification to the way these errors are dealt with should be done homogeneously across the various methods.
2. Tests should be added to cover the lines dealing with these parsing errors. That will require a special LHE file, if not several.

[Eduardo Rodrigues]

This work should probably be done taking #97 into account, to finalise on it.