[Mne_analysis] epochs should throw an error if labels doesn't exist

[Dan Howarth]

Let's say you have an epochs object which you believe has labels 'foo' and
'bar', but really 'bar' doesn't exist as a label in the object.

epochs['bar'].average() throws an error, which seems reasonable

epochs[['foo', 'bar']].average() does not throw an error and might be
responsible for creating a whole field of brain science based on false
results ;)

This is 0.13.1

-Dan
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