Present the actual error message from raise_error

[genehsu]

Fixes #1190

Display the actual error message of the raised error when a raise_error assertion fails. The failure message will now include the error message on a separate line followed by the backtrace. If there is no error message, only the backtrace will be shown.

If this approach is not correct, then please close this PR without prejudice 😃

[genehsu]

@JonRowe

Thanks for tackling this, can you check out these couple of suggestions?

Thanks for the review. I believe I've addressed your suggestions with clearer code. Let me know if you have any other suggestions to the code or output.

[JonRowe]

Migrated to the monorepo as rspec/rspec#128