[mlir][MemRef][TransformOps] Fix error reporting for multibuffer

Multibuffer will fail to apply on allocs that are used outside of loops.
This was properly caught in the current implementation but the way we report
it was broken.
Notes cannot be emitted on their own, they need to be attached to another
main diagnostic.

Long story short, change the severity of the report from Note to Error.

Differential Revision: https://reviews.llvm.org/D143729