Issue #502 - Add tests and Improve error handling

[nikosbosse]

Description

This PR closes #502.

Replacing warnings with errors:

• bias_quantile()
• interval_coverage
• interval_coverage_deviation()

This makes it more consistent with the behaviour of other functions that also error. In addition, we get a nicer message in score(), where the error gets converted into a warning like Error in 'interval_coverage()': error content.

Further updates to bias_quantile():

• update docs
• improve error handling if median isn't there and can't be imputed

updates tests for the above

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[seabbs]

/document

[seabbs]

Nice!

[seabbs]

I may have some issues with all the rebasing here and if so I can investigate

[nikosbosse]

I may have some issues with all the rebasing here and if so I can investigate
That was just due to the fact that one PR updated a test for a function that another PR deleted