Exception in get_group() with invalid index
Hello,

I'm using the pandas function "pandas.core.groupby.GroupBy.get_group()" to parallelize my code.
This function raises a "KeyError" exception if the passed index is not contained in the index list of the grouped DataFrame.

A basic example is:
data = DataFrame({'ind':[0,0,2,2]})
gb=a.groupby('ind')
gb.get_group(1)

Which raises "KeyError: 1"

In order to use intensively the "get_group()" function in code parallelization, an option to avoid this exception would be very useful, similar to "error_bad_lines=False" of the "read_csv()" function.

An empty DataFrame could be returned by the "get_group()" function when this option and an invalid index are given.
