Improve raise_error messages

[mcmire]

This commit makes some minor changes to the raise_error matcher to make the failure messages slightly more friendly and consistent with the overall philosophy of matchers in this repo.

Here are the current messages in comparison with the new ones:

• In cases where the block raises no error, the wording changes from "Expected block" to "Expected exception-free block", and "exception-free block" is highlighted in the actual color. This is to emphasize the actual behavior vs. the expected behavior and matches other matchers.
• In cases where only an expected error class is provided, the wording changes so that instead of inspecting the expected error, the failure message says "a kind of" followed by the class name.
• In cases where a message is provided, the expected class and message are separated in the failure message.
• In cases where a regexp message is provided, the wording changes from "with message #{message}" to "with message matching #{message}".

Additionally, the integration tests for raise_error have been reviewed and missing tests have been filled in.