Better error msg for malformed freq alias
```
In [17]: date_range(datetime.datetime.today(), periods=10, freq='2h20m')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
/home/chang/Dropbox/git/pandas/<ipython-input-17-ff4e03382573> in <module>()
----> 1 date_range(datetime.datetime.today(), periods=10, freq='2h20m')

/home/chang/Dropbox/git/pandas/pandas/tseries/index.pyc in date_range(start, end, periods, freq, tz, normalize)
   1209     """
   1210     return DatetimeIndex(start=start, end=end, periods=periods,
-> 1211                          freq=freq, tz=tz, normalize=normalize)
   1212 
   1213 

/home/chang/Dropbox/git/pandas/pandas/tseries/index.pyc in __new__(cls, data, freq, start, end, periods, copy, name, tz, verify_integrity, normalize, **kwds)
    202 
    203         if data is None and offset is None:
--> 204             raise ValueError("Must provide freq argument if no data is "
    205                              "supplied")
    206 

ValueError: Must provide freq argument if no data is supplied
```
