Include line number in error messages

When an error is encountered while parsing a DON, don.Error is
raised with an error message that describes the error. Include
the source of DON and line number, where the error occurred while
parsing the DON, in the error message. The source is either
'<str>' when the DON is specified as a string or the path to a
file if a path to a DON file is specified while parsing DON.