Introduce concept of parse errors for quick actions

What does this MR do and why?

See #352001 (closed), #352002 (closed)

This fixes issues with user argument parsing in quick actions (such as assign/unassign) by verifying that we find one user for each argument.

It introduces a mechanism for reporting parameter parse errors in quick actions which aborts processing.

In order to introduce stricter argument validation, some commands that were previously acceptable are now rejected: for example the commands below would work before these changes:

/assign me to #123
/unassign @good_user @bad_user
/unassign @someone /assign @someone_else

Not that they will necessarily do what you want!

Now they will all be rejected, which overall I think is safer, and will result in fewer surprises.

How to set up and validate locally

See linked issues, which include reproduction steps.

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