BUG: Fix error during benchmark update over empty period.

On ranges with missing data from Yahoo, e.g.:
On 2013-04-2 the date range of April 2013-03-29 failed because
of the first day in the range being Good Friday, and the API not
yet updating for the Monday after.

Handle the 404 that is found by raising and warning that no
benchmark data was found, but continuing on.