NLM3xml.py: include reference file in error log message

[ehenneken]

When an unknown citation type is encountered, the NLM3xml.py parser reports this in the error log:

if type not in self.types:
            logger.error("NLMxml: found unknown reference type '%s'" % type)
            pass

The reference will not get skipped, but the error message does not include the reference file where this was found. If the reference type reported is a legitimate reference type, we need to update the parser. However, in all other cases it is a data problem and we need to know where to fix the data problem.