Add line numbers to xml validation error messages

[csc-jm-zz]

Proposed solution

Currently XMLSchemaValidationError exceptions raised by xmlschema library do not include specific line numbers of the error occurrence in the XML file. Including the line number in the error reason could be useful information for users and ParseError exceptions already include the position in the detail.reason that gets returned upon validation error.

@otahontas mentioned in #88 that one way to do this would be:

import lxml.etree as etree
root = etree.parse("XPTO.XML").getroot()
errors = [e for e in schema.iter_errors(root)]

DoD (Definition of Done)

detail.reason generated in xml validation method includes line number for any error case.

As an example, validating SRP000539_invalid6.xml could return:
Unexpected child with tag 'BAD_ELEMENT' at position 2, line 34.

Testing

Unit tests

[blankdots]

might be fixed with: sissaschool/xmlschema#164