feat: add `failure_message` option for tasks (#2080)

[dusktreader]

• Do not show stack trace on task failure
• If a failure_message option is provided, show that message: "Task N failed: a nice failure message"
• If no failure_message option is provided, show the subprocess exception: "Task N failed: Command 'might-fail' returned non-zero exit status 1."

Addresses Issue #2080: https://github.com/copier-org/copier/issues/2080

[dusktreader]

I rebased this branch on tsvikas/feat/task-error branch and updated it to work with that change.

[dusktreader]

@sisp I have rebased on master. Let me know if any other adjustments are needed.