updated error message for obsolete vars

[clmould]

Description

Added to the error message for obsolete variables in the input file to state what their new variable name is from obsolete_vars.py as previously this was not output.

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[jonmaddock]

Looks great Clair! Can you just rebase (fixup) these two commits to reduce the churn (doing and undoing) into a single commit?

[jonmaddock]

Looks great!