[Feature]: in bundless, if cannot resolve, error message should be emitted

[SoonIter]

          By the way, in bundless mode, if cannot resolve, error message should be emitted

Originally posted by @SoonIter in #250 (comment)

[fi3ework]

if cannot resolve, error message should be emitted

In this case, it's because the resolved module is externalized, Rspack won't check if it's available anymore.

Yes, we need to ensure that the emitted files could be resolved. In this case, we overlooked the ⁠index mainFields, which makes this a fixable bug for us. Let's see if there are any paths that Rslib could not detect in the future. If so, we can add a post-check.