Display iteration variable names in solver error message

[clmould]

Description

Improve error message when x is initially in an infeasible region to explicitly state the variable names of the iteration variables instead of just their number to help with debugging

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.