Flaky external websites shouldn't fail the build

[ChrisJohnNOAA]

Describe the bug
Currently external sites that are not available (either temporarily or permanently) and are linked to from ERDDAP pages will cause the ERDDAP build to fail. While it is useful to know if external links are broken, failing the build is not the proper solution. A failing build should indicate ERDDAP internal problems.

This is a separate issue from flaky external data sources that are used in tests.

To Reproduce
See an example error here: https://github.com/ERDDAP/erddap/actions/runs/12890220698/job/35939294125

Expected behavior
Internal broken links can cause a failure in ERDDAP tests, but external links should not cause the build to fail. Broken external links should probably be a separate tool to run with it's own output.