Updating generic.py error message

This is my first attempt at a PR. I just made a little change to allow an error message. Would appreciate some feedback. 
Based on https://github.com/pydata/pandas/issues/8015
I tried that and then I get broken tests. Any suggestions?
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FAIL: test_fails_on_no_datetime_index (pandas.tseries.tests.test_resample.TestTimeGrouper)
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Traceback (most recent call last):
  File "/home/peadarcoyle/Code/pandas/pandas/tseries/tests/test_resample.py", line 1385, in test_fails_on_no_datetime_index
    df.groupby(TimeGrouper('D'))
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1604, in __exit__
    return self.handle_success(exc_type, exc_value, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1617, in handle_success
    raise_with_traceback(e, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/tseries/tests/test_resample.py", line 1385, in test_fails_on_no_datetime_index
    df.groupby(TimeGrouper('D'))
  File "/home/peadarcoyle/Code/pandas/pandas/core/generic.py", line 2876, in groupby
    raise TypeError('You have to specify one of "by" or "level"')
AssertionError: "axis must be a DatetimeIndex, but got an instance of 'Int64Index'" does not match "You have to specify one of "by" or "level""

======================================================================
FAIL: test_insert_error_msmgs (pandas.tests.test_frame.TestDataFrame)
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Traceback (most recent call last):
  File "/home/peadarcoyle/Code/pandas/pandas/tests/test_frame.py", line 2661, in test_insert_error_msmgs
    df['gr'] = df.groupby(['b', 'c']).count()
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1604, in __exit__
    return self.handle_success(exc_type, exc_value, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1617, in handle_success
    raise_with_traceback(e, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/tests/test_frame.py", line 2661, in test_insert_error_msmgs
    df['gr'] = df.groupby(['b', 'c']).count()
  File "/home/peadarcoyle/Code/pandas/pandas/core/generic.py", line 2876, in groupby
    raise TypeError('You have to specify one of "by" or "level"')
AssertionError: "incompatible index of inserted column with frame index" does not match "You have to specify one of "by" or "level""

======================================================================
FAIL: test_filter_enforces_scalarness (pandas.tests.test_groupby.TestGroupBy)
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Traceback (most recent call last):
  File "/home/peadarcoyle/Code/pandas/pandas/tests/test_groupby.py", line 4251, in test_filter_enforces_scalarness
    df.groupby('c').filter(lambda g: g['a'] == 'best')
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1604, in __exit__
    return self.handle_success(exc_type, exc_value, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1617, in handle_success
    raise_with_traceback(e, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/tests/test_groupby.py", line 4251, in test_filter_enforces_scalarness
    df.groupby('c').filter(lambda g: g['a'] == 'best')
  File "/home/peadarcoyle/Code/pandas/pandas/core/generic.py", line 2876, in groupby
    raise TypeError('You have to specify one of "by" or "level"')
AssertionError: "filter function returned a.*" does not match "You have to specify one of "by" or "level""

======================================================================
FAIL: test_filter_non_bool_raises (pandas.tests.test_groupby.TestGroupBy)
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Traceback (most recent call last):
  File "/home/peadarcoyle/Code/pandas/pandas/tests/test_groupby.py", line 4264, in test_filter_non_bool_raises
    df.groupby('a').filter(lambda g: g.c.mean())
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1604, in __exit__
    return self.handle_success(exc_type, exc_value, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/util/testing.py", line 1617, in handle_success
    raise_with_traceback(e, traceback)
  File "/home/peadarcoyle/Code/pandas/pandas/tests/test_groupby.py", line 4264, in test_filter_non_bool_raises
    df.groupby('a').filter(lambda g: g.c.mean())
  File "/home/peadarcoyle/Code/pandas/pandas/core/generic.py", line 2876, in groupby
    raise TypeError('You have to specify one of "by" or "level"')
AssertionError: "filter function returned a.*" does not match "You have to specify one of "by" or "level""