Prevent crashing with malformed date

[nibrahim]

% invoice invoice ls --from=01/04/2017
Traceback (most recent call last):
File "/home/noufal/projects/python/environments/invoices/bin/invoice", line 9, in
load_entry_point('invoice==0.1.1a0', 'console_scripts', 'invoice')()
File "/home/noufal/projects/python/environments/invoices/lib/python3.4/site-packages/invoice-0.1.1a0-py3.4.egg/invoice/invoice.py", line 305, in main
dispatch(args)
File "/home/noufal/projects/python/environments/invoices/lib/python3.4/site-packages/invoice-0.1.1a0-py3.4.egg/invoice/invoice.py", line 295, in dispatch
command_handler()
File "/home/noufal/projects/python/environments/invoices/lib/python3.4/site-packages/invoice-0.1.1a0-py3.4.egg/invoice/commands.py", line 58, in call
return sc_handler()
File "/home/noufal/projects/python/environments/invoices/lib/python3.4/site-packages/invoice-0.1.1a0-py3.4.egg/invoice/commands.py", line 499, in list
from_ = None if self.args['from'] == 'a' else datetime.datetime.strptime(self.args['from'], "%d/%b/%Y")
File "/usr/lib/python3.4/_strptime.py", line 500, in _strptime_datetime
tt, fraction = _strptime(data_string, format)
File "/usr/lib/python3.4/_strptime.py", line 337, in _strptime
(data_string, format))
ValueError: time data '01/04/2017' does not match format '%d/%b/%Y'