error reporting: a better way of reporting progress/error to Tui/GUI

[oliver-sanders]

Placeholder issue for work to improve the communication of command progress / error to Tui/GUI and any other apps which may care to use the CLI as an interface.

Why use the CLI rather than call Python directly:

For some commands we may need to be able to change the Cylc version dynamically. E.G. The UIs needs to be able to upgrade workflows on the hoof, i.e. stop then restart them with a higher version of Cylc. The UIS might also need to be able to restart a workflow with a lower version of Cylc than the UIS itself, e.g. for production purposes.

For other commands, the desire to change Cylc version of the command run isn't so important.

How to pass error:

• zmq: implement REQ-REP + PUB-SUB comms #3329 will provide us with the ability to "stream" command progress to the client. We could use log levels to allow the client to filter out the important information.
• Stdout/err: stdout too chatty, stderr too concise?
• Other?