Only handle status errors when sendError is called

Update ErrorPageFilter to only handle errors when `response.sendError`
has been called. This should allow custom @exceptionhandlers to
completely handle errors and return custom status codes without
triggering the "Cannot forward to error page" log message.

The Javadoc for sendError states:

  "The server defaults to creating the response to look like an
   HTML-formatted server error page containing the specified message"

Where as setStatus states

  "This method is used to set the return status code when there is
   no error "

Fixes gh-2745

Author: philwebb

spring-boot/src/main/java/org/springframework/boot/context/web/ErrorPageFilter.java

@@ -114,9 +114,9 @@ private void doFilter(HttpServletRequest request, HttpServletResponse response,
 		ErrorWrapperResponse wrapped = new ErrorWrapperResponse(response);
 		try {
 			chain.doFilter(request, wrapped);
-			int status = wrapped.getStatus();
-			if (status >= 400) {
-				handleErrorStatus(request, response, status, wrapped.getMessage());
+			if (wrapped.hasErrorToSend()) {
+				handleErrorStatus(request, response, wrapped.getStatus(),
+						wrapped.getMessage());
 				response.flushBuffer();
 			}
 			else if (!request.isAsyncStarted() && !response.isCommitted()) {
@@ -140,7 +140,6 @@ private void handleErrorStatus(HttpServletRequest request,
 			handleCommittedResponse(request, null);
 			return;
 		}
-
 		String errorPath = getErrorPath(this.statuses, status);
 		if (errorPath == null) {
 			response.sendError(status, message);
@@ -321,6 +320,10 @@ public String getMessage() {
 			return this.message;
 		}
 
+		public boolean hasErrorToSend() {
+			return this.hasErrorToSend;
+		}
+
 	}
 
 }