On new event, with -i flag, daterange parsing error shouldn't wipe the whole event

If I screw up the datetime range when adding a new event, khal should...

1. give me a clue about what was wrong with the range given (same as #786 ), and also
2. give me a chance to revise the datetime range, without starting over.

Right now if I screw up, I have to add the whole event from scratch:

amanda@mona:~$ khal new -i
calendar: logistics
summary: Coffee w/Somebody at a Place
datetime range: tuesday 13:30 - 14:30
Traceback (most recent call last):
File "/usr/local/bin/khal", line 11, in
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 396, in new
format=format,
File "/home/amanda/.local/lib/python3.5/site-packages/khal/controllers.py", line 301, in new_interactive
daterange, conf['locale'], adjust_reasonably=True)
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 tuesday 13:30 - 14:30 as a daterange
amanda@mona:~$

Start at khal/controllers.py in new_interactive, which calls khal/utils.py:guessrangefstr; the command entry point is khal/cli.py. Reproduce the invalid datetime range with khal new -i, then verify that the range error is explained and the existing event fields remain available for another range attempt.