Use request attribute if available for error message

Fixes gh-1762, gh-1731

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/DefaultErrorAttributes.java

@@ -75,6 +75,7 @@ public class DefaultErrorAttributes implements ErrorAttributes, HandlerException
 
 	private void storeErrorAttributes(HttpServletRequest request, Exception ex) {
 		request.setAttribute(ERROR_ATTRIBUTE, ex);
+		
 	}
 
 	@Override
@@ -120,7 +121,7 @@ public class DefaultErrorAttributes implements ErrorAttributes, HandlerException
 				addStackTrace(errorAttributes, error);
 			}
 		}
-		else {
+		if (error==null || errorAttributes.get("message")==null) {
 			Object message = getAttribute(requestAttributes,
 					"javax.servlet.error.message");
 			errorAttributes.put("message", message == null ? "No message available"

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/DefaultErrorAttributesTests.java

@@ -118,6 +118,17 @@ public class DefaultErrorAttributesTests {
 		assertThat(attributes.get("message"), equalTo((Object) "Test"));
 	}
 
+	@Test
+	public void nullMessage() throws Exception {
+		this.request.setAttribute("javax.servlet.error.exception", new RuntimeException());
+		this.request.setAttribute("javax.servlet.error.message", "Test");
+		Map<String, Object> attributes = this.errorAttributes.getErrorAttributes(
+				this.requestAttributes, false);
+		assertThat(attributes.get("exception"),
+				equalTo((Object) RuntimeException.class.getName()));
+		assertThat(attributes.get("message"), equalTo((Object) "Test"));
+	}
+
 	@Test
 	public void unwrapServletException() throws Exception {
 		RuntimeException ex = new RuntimeException("Test");