Better error message when date_range improperly specified
noticed in test failure:

```
======================================================================
ERROR: test_shift_multiple_of_same_base (__main__.TestLegacySupport)
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Traceback (most recent call last):
  File "pandas/tests/test_timeseries.py", line 624, in test_shift_multiple_of_same_base
    ts = Series(np.random.randn(5), index=date_range('1/1/2000', freq='H'))
  File "/home/wesm/code/pandas/pandas/core/daterange.py", line 65, in date_range
    freq=freq, tz=tz, normalize=normalize)
  File "/home/wesm/code/pandas/pandas/core/index.py", line 1279, in __new__
    b, e = Timestamp(start), Timestamp(end)
  File "datetime.pyx", line 59, in pandas._tseries.Timestamp.__new__ (pandas/src/tseries.c:27720)
AttributeError: 'NoneType' object has no attribute 'dtval'

----------------------------------------------------------------------
```
