Add parsing input to QtyError messages

[wvteijlingen]

Something parsing fails and a QtyError is thrown. The error messages give a vague idea of what went wrong, but it would be great if they could include the input that it was trying to parse. That way you can easily glance from the error messages if for for example the input format is not supported.

Example: Unit not recognized becomes Unit not recognized when attempting to parse "1 LT".