daterange parsing error should include a hint

If I use the wrong formatting to ask for a list of events on some given future data, eg khal list 18-june-2018 10d I get a heap of errors that end with the very clear notice that khal wasn't able to parse the date range I provided:

amanda@mona:~$ khal list 18-june-2018 10d
Traceback (most recent call last):
  File "/usr/local/bin/khal", line 11, in <module>
    sys.exit(main_khal())
  File "/home/amanda/.local/lib/python3.5/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/home/amanda/.local/lib/python3.5/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/home/amanda/.local/lib/python3.5/site-packages/click/core.py", line 1066, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/amanda/.local/lib/python3.5/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/amanda/.local/lib/python3.5/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/home/amanda/.local/lib/python3.5/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/amanda/.local/lib/python3.5/site-packages/khal/cli.py", line 323, in klist
    env={"calendars": ctx.obj['conf']['calendars']}
  File "/home/amanda/.local/lib/python3.5/site-packages/khal/controllers.py", line 226, in khal_list
    default_timedelta_datetime=conf['default']['timedelta'],
  File "/home/amanda/.local/lib/python3.5/site-packages/khal/controllers.py", line 142, in start_end_from_daterange
    default_timedelta_datetime=default_timedelta_datetime,
  File "/home/amanda/.local/lib/python3.5/site-packages/khal/utils.py", line 438, in guessrangefstr
    raise ValueError('Could not parse `{}` as a daterange'.format(daterange))
ValueError: Could not parse `('18-june-2018', '10d')` as a daterange

Ideally khal would not show me the full traceback, but would be more precise about what it is looking for in a date. Something like:

ValueError: Could not parse ('18-june-2018', '10d') as a daterange -- "18-june-2018" is not a valid date. Khal expects dates in the format "18-may" Check the usage documentation for more examples of START and END date formats.

If I give a bad DELTA, eg. khal calendar 18-jun 1w I get a slightly different error, but it doesn't tell me that the delta is the problem:

critical: Could not parse ('18-jun', '1w') as a daterange

Something like "10w does not appear to be a valid END or DELTA value" would clue me into the fact that khal doesn't do weeks.

Start with khal/utils.py at guessrangefstr and trace its callers through controllers.py at start_end_from_daterange. Reproduce the invalid date and delta examples first, then check the usage documentation for the expected formats. Done means date-range errors identify whether the date or delta is invalid and provide a useful hint instead of an unhelpful traceback.