Tidy error path through local channel execute_no_wait
Tidy error path through local channel execute_no_wait

Prior to this commit, if an execution was thrown inside the main
part of the body, pid might not have been assigned yet, resulting in
this exception:

> UnboundLocalError: local variable 'pid' referenced before assignment

This commit changes behaviour so that the original exception is
propagated, after logging.

swift-lang/swift-e-lab,Parsl/parsl,Parsl/parsl,swift-lang/swift-e-lab,Parsl/parsl,Parsl/parsl