ENH: Raise error writing excel file with a MultiIndexed DataFrame #9794
Raise a NotImplementedError until fixed.

closes #9794

can you add a tests that validates this. otherwise looks good.
 Actually I just saw there is a some tests that test writing excel with multiindex

https://github.com/pydata/pandas/blob/master/pandas/io/tests/test_excel.py#L895

Referencing:
#5616

ill figure out what the difference is
 I added two tests that make sure writing MultiIndex index works and MultiIndex columns raise.
I added the assertRaises to the tests that failed
 fixed the backticks in whats new and modified the test for multicolumn/index writing to assert fails only on multicolumn writes
 @hsperr thanks!
