fix: Drop container error to warning

Changes:
 - Container registries do not have a consistent API, so calling them
   for container presence doesn't seem to work on GHCR or NVCR.
 - This drops the error to a warning so we don't fail tests.

Relates to nf-core/modules#3693 and nf-core#2393

CHANGELOG.md

@@ -9,6 +9,7 @@
 - Add module MULTIQC to modules.config ([#2377](https://github.com/nf-core/tools/pull/2377))
 - Update the Code of Conduct ([#2381](https://github.com/nf-core/tools/pull/2381))
 - Save template information to `.nf-core.yml` and deprecate argument `--template-yaml` for `nf-core sync` ([#2388](https://github.com/nf-core/tools/pull/2388) and [#2389](https://github.com/nf-core/tools/pull/2389))
+- Reduce container finding error to warning since the registries are not consistent.
 
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