Mininet should detect when mnexec fails

If mnexec fails or isn't working correctly, Mininet hangs and doesn't start up! :(

We should try to detect this and quit with an error message including the error output of mnexec.

It's tricky because it's hard to tell the difference between mnexec hanging and simply taking a long time to start up. However, if it has exited before returning any reasonable output to us, we know it has failed.