Fix panic when failing to push images

[laozc]

A panic might happen when the auth required by a repo is not set. This is due to the program tries to write to a channel which might have been closed.

The current logic tries to use one channel for result and another channel for any possible errors. However, when routine exists, the error might be written to the error channel after both channels might have been closed.

I changed the code to use only one channel.