Fix "Generator already running" error

this is the specific case that creates the "generator already running" error. i extracted it from the other PR because that PR got too convoluted, and this occurs whether or not there is a callback supplied.

note: i would consider this a developer error (syntax error or reference error). we could just throw it instead. we could also make a "fast" and "slow" version of co and only do this in the "slow" version (i.e. development).