make RoseStemVersionException print the correct missing variable

[wxtim]

Fixes a small bug in an error printout:

How to reproduce:

mkdir -p "${HOME}/example/rose-stem"
touch "${HOME}/example/rose-stem/flow.cylc"
touch "${HOME}/example/rose-stem/rose-suite.conf"

rose stem \
    --source=foo="${HOME}/example"\
    --workflow-name=$(uuidgen)

Will return an error, but at the moment the error is:

RoseStemVersionException: Running rose-stem version 1 but suite is does not have ROSE_VERSION set in the rose-suite.conf

But this is wrong and unhelpful: The variable should be ROSE_STEM_VERSION.

Additionally I've made RoseStemVersionException inherit CylcError so that we can suppress the traceback, unless in debug mode.

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included Any test of this change would either be overkill, or trivial.
• CHANGES.md entry included This is effectively an internal documentation change.
• ~Cylc-Doc ~ This is related to documentation changes but does not itself require them.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.