BUG#128: Better error messages for invalid column references in concat

[luketebbs]

A schema with an invalid column reference inside concat doesn't give helpful error messages at present. With this change applied -

version 1.1
@totalColumns 3
c1:
c2:
c3: is(concat($c2,$c4))

will now give an error message saying 'Column: c3 has invalid cross reference is(concat($c2, $c4)) at line: 5, column: 5' rather than just 'Invalid column definition'.