throw informative error for quantile regression glance with multiple tau values; closes #330

[bfgray3]

I've tried to close #330, whose underlying problem was originally brought up in #206. I updated the roxygen but I was getting the following error when trying to document and I'm not sure why.

> devtools::document()
Updating broom documentation
Loading broom
Warning: @details [rq_tidiers.R#82]: mismatched braces or quotes

[alexpghayes]

Odd. AppVeyor seems happy and my local build was also fine. I'm going to merge and if something explodes deal with it then.

Thanks for the PR!

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