Support farm selection on imports and training processes

Closes #1325 (closed), #1326 (closed)

I had started adding a specific handling for farm errors, but found that implementing the farm selection first, and then adding error handling using the same method used by the process workers page was a lot faster. Adding that to the training process and testing it took me five minutes.

While testing this, I also fixed a facepalm-inducing mistake in the process workers page, which was causing an array to be displayed as JSON instead of displaying the error message itself: