Cancelled state for arq scheduler?

[Peter9192]

Arq does not have a "cancelled" state according to https://github.com/samuelcolvin/arq/blob/main/arq/jobs.py#L25-L39

At the moment jobs that error out due to full disk, permission, service down, etc. or jobs that are cancelled explicitly by a user both set to 'error' state.

It could be useful to distinguish the two.

Originally posted by @sverhoeven in #20 (comment)