ENH: throw exceptions at generator
ENH: throw exceptions at generator

If the run engine has an exception raised during
 - look up of the coroutine to run
 - running the coroutine

throw the exception back at the generator.  This gives two
major features:

 - tracebacks will include the line in the generator which yielded the
   bad message
 - allows generators to attempt to handle the excetion without killing
   the run.

ericdill/bluesky,ericdill/bluesky