Fix Issue #1855: "All Bazel commands fail when changing a Go dependency when using Bzlmod extension"

[Buzz-Lightyear]

What type of PR is this?
Bug fix

What package or component does this PR mostly affect?
Go bzlmod

What does this PR do? Why is it needed?
When trying to bump the version of a Go module in a repo that uses the go_deps bzlmod extension, we run into an issue wherein all Bazel commands fail with "Error in fail: No sum for <module>. However the suggested fix is to run bazel run @rules_go//go -- mod tidy which fails with the same error, thus forcing us into a pickle.

This PR changes this to a warning instead of an error after which the suggested fix works.

Which issues(s) does this PR fix?

Fixes #1855

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