Rollup merge of #147395 - reddevilmidzy:refact-error-msg, r=Kivooeo,jackh726

Improve diagnostics: update note and add help message

I moved the content from the note to a help message, as it seemed more appropriate there, and then added new information to the note(`Modules are usually placed outside of blocks, at the top level of the file`)!

resolve: rust-lang/rust#147314

Author: matthiaskrgr