I just wanted to remove CommandError from parse_immutable_expression(), which
will be called from the templater, but the new error message looks also better.
Some of them will be called directly from the commit templater which shouldn't
know WorkspaceCommandHelper. All parameters are passed as function arguments
instead of having a nicer wrapper struct. That's because some resources (e.g.
repo and id prefix context) are also used for different purposes, and it seemed
uneasy to introduce high-level abstraction satisfying all the use cases.
