BUG:  Holiday(..) with both offset and observance raises
Resolves #10217 

Doesn't fix the underlying issue yet, but at least raises on the unsupported behavior.

pls add a release note (put it in the API section for 0.17.0). minor changes otherwise. pls ping when green on travis.
 As requested. By the way, five points to whoever wrote the 'contributing to pandas' doc section.
 thanks to 

cc @rockg who put most of that together!
 cc @rockg

look good to you?
 Looks good to me.  Let's keep the original issue open until that requested behavior is supported.

@bwillers Glad it was useful...hopefully removes the major hurdles to helping out.
 merged via https://github.com/pydata/pandas/commit/d513986e75110b14e288753701e435660f3d115e

thanks!
