setup.py: fail with exception when bower cannot be found.
setup.py: fail with exception when bower cannot be found.

SylvainCorlay/bqplot,bloomberg/bqplot,ssunkara1/bqplot,dmadeka/bqplot,bloomberg/bqplot,ssunkara1/bqplot,dmadeka/bqplot,ChakriCherukuri/bqplot,rmenegaux/bqplot,SylvainCorlay/bqplot,rmenegaux/bqplot,SylvainCorlay/bqplot,ChakriCherukuri/bqplot,bloomberg/bqplot,ChakriCherukuri/bqplot