BUG: fix broken BigqueryError reference
`BigqueryError` wasn't imported directly, and so has to be referred to via the module.

was this another cython bug finder?
 Yeah, I have a handful of them.
 Is there a way to get Travis to try again?
 git commit --amand -C HEAD

force push 

there are rebuild buttons as well on the the of the Travis screen
 @jreback: thanks, pushing the amended version seems to work. 
 thanks for the catch!
