Prior to this commit, `MockMvc` would support checking for the Servlet
error message as the "response status reason". While this error message
can be driven with the `@ResponseStatus` annotation, this message is not
technically the HTTP status reason listed on the response status line.
This message is provided by the Servlet container in the error page when
the `response.sendError(int, String)` method is used.
This commit adds the missing
`mvc.get().uri("/error/message")).hasErrorMessage("error message")`
assertion to check for this Servlet error message.
Closes gh-34016
52006b71bc