Support Servlet error message in MockMvc assertions

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

spring-test/src/main/java/org/springframework/test/web/servlet/assertj/AbstractMockHttpServletResponseAssert.java

@@ -22,6 +22,7 @@ import org.assertj.core.api.AbstractByteArrayAssert;
 import org.assertj.core.api.AbstractStringAssert;
 import org.assertj.core.api.Assertions;
 import org.assertj.core.api.ByteArrayAssert;
+import org.assertj.core.api.StringAssert;
 
 import org.springframework.lang.Nullable;
 import org.springframework.mock.web.MockHttpServletResponse;
@@ -163,4 +164,16 @@ public abstract class AbstractMockHttpServletResponseAssert<SELF extends Abstrac
 		return this.myself;
 	}
 
+	/**
+	 * Verify that the {@link jakarta.servlet.http.HttpServletResponse#sendError(int, String)} Servlet error message}
+	 * is equal to the given value.
+	 * @param errorMessage the expected Servlet error message (can be null)
+	 * @since 6.2.1
+	 */
+	public SELF hasErrorMessage(@Nullable String errorMessage) {
+		new StringAssert(getResponse().getErrorMessage())
+				.as("Servlet error message").isEqualTo(errorMessage);
+		return this.myself;
+	}
+
 }