Improve custom command error handling - check for running project or service

[andrewfrench]

Is your feature request related to a problem? Please describe.
When a custom command targets a non-running container, the error is fairly opaque:

Failed to run <cmd> : failed to load any docker-compose.*y*l files: <nil>

Describe the solution you'd like
An error message that describes the issue more generally:

Failed to run <cmd>: <project> is not running

[rfay]

Thanks for this! Maybe can clean up the similar stuff that comes out when you stop an already-stopped project.

Another approach on this issue is to not run custom commands at all if the project isn't started?