typo in variable name needs good error message

[ahelsing]

My code had a typo, but QGL2 gave no good error; provide a good error message with line number:

            X90m(qubi, dragScaling=dragScaling)

And I got a nasty stacktrace:

2019-06-18 19:44:33.314327: CALLING EVALUATOR
Traceback (most recent call last):
  File "./FlipFlop.py", line 153, in <module>
    main()
  File "./FlipFlop.py", line 128, in main
    (q, np.linspace(0, 5e-6, 11), 10))
  File "/home/ahelsing/Projects/Quantum/pyqgl2/src/python/pyqgl2/main.py", line 278, in compile_function
    ptree1 = evaluator.visit(ptree1)
  File "/home/ahelsing/Projects/Quantum/pyqgl2/src/python/pyqgl2/eval.py", line 954, in visit
    node.body = self.do_body(node.body)
  File "/home/ahelsing/Projects/Quantum/pyqgl2/src/python/pyqgl2/eval.py", line 1684, in do_body
    success, new_stmnts = self.do_for(stmnt)
  File "/home/ahelsing/Projects/Quantum/pyqgl2/src/python/pyqgl2/eval.py", line 1172, in do_for
    new_body = self.do_body(new_body)
  File "/home/ahelsing/Projects/Quantum/pyqgl2/src/python/pyqgl2/eval.py", line 1684, in do_body
    success, new_stmnts = self.do_for(stmnt)
....
    self.call_checker(stmnt.value.qgl2_check_vector)
  File "/home/ahelsing/Projects/Quantum/pyqgl2/src/python/pyqgl2/eval.py", line 1465, in call_checker
    value = local_variables[mapped_name]
KeyError: 'qubi'