[MLIR][OpenMP] Better error reporting for unsupported `nowait` (#78551)

Provides some context for failing to generate LLVM IR for `target
enter|exit|update` directives when `nowait` is provided. This is
directly helpful for flang users since they would get this error message
if they tried to use `nowait`. Before that we had a very generic
message.

This is a follow-up to https://github.com/llvm/llvm-project/pull/78269,
please only review the latest commit (the one with the same commit
message as the PR title).