.roll/.dice breaks if called without arguments.

[creftos]

An exception is thrown if you try to call .roll or .dice:

(4:57:10 PM) creftos: .dice
(4:57:10 PM) Willie: TypeError: expected string or buffer (file "/usr/lib64/python2.7/re.py", line 177, in findall)

And in the logs:

    Traceback (most recent call last):
      File "/home/bdc/workspace/willie/willie/bot.py", line 741, in call
        exit_code = func(willie, trigger)
      File "/home/bdc/workspace/willie/willie/modules/dice.py", line 175, in roll
        dice_expressions = re.findall(dice_regexp, arg_str)
      File "/usr/lib64/python2.7/re.py", line 177, in findall
        return _compile(pattern, flags).findall(string)
    TypeError: expected string or buffer

Instead of throwing an exception, Willie should fail gracefully and inform the user of the kind of input that is expected.

Additional Information:
Willie v. 4.5.1
OS: Fedora release 20 (Heisenbug)
Kernel Version: 3.16.2-201.fc20.x86_64