Make _.first() assertions consistant with _.last()'s
Make _.first() assertions consistant with _.last()'s

In pull request #2360 @michaelficarra suggested an improvement to the assertion
messages for `_.last()`'s two `n <= 0` cases. This change brings `_.first()`
assertions in line with this improved format.

akre54/underscore,tnga/underscore-es,huangnan1/2016gitStudy,huangnan1/2016gitStudy,keepgoingwm/underscore,keepgoingwm/underscore,tnga/underscore-es,rlugojr/underscore,jdelbello/underscore,vincent1988/underscore,documentcloud/underscore,jpalomar/underscore,documentcloud/underscore,rlugojr/underscore,vincent1988/underscore,jpalomar/underscore,jashkenas/underscore,akre54/underscore,jdelbello/underscore,jashkenas/underscore,platinumazure/underscore,platinumazure/underscore