ESP8266HTTPUpdateServer: fix responses after uploading

- fix response not being delivered to the browser after upload is done
  (https://github.com/esp8266/Arduino/issues/2221)

- if Update.begin fails, don’t attempt to write data

- if update is not successful, send error message from Update to the
  client

- move strings into PROGMEM

libraries/ESP8266HTTPUpdateServer/src/ESP8266HTTPUpdateServer.h

@@ -5,15 +5,6 @@ class ESP8266WebServer;
 
 class ESP8266HTTPUpdateServer
 {
-  private:
-    bool _serial_output;
-    ESP8266WebServer *_server;
-    static const char *_serverIndex;
-    static const char *_failedResponse;
-    static const char *_successResponse;
-    char * _username;
-    char * _password;
-    bool _authenticated;
   public:
     ESP8266HTTPUpdateServer(bool serial_debug=false);
 
@@ -33,6 +24,17 @@ class ESP8266HTTPUpdateServer
     }
 
     void setup(ESP8266WebServer *server, const char * path, const char * username, const char * password);
+
+  protected:
+    void _setUpdaterError();
+
+  private:
+    bool _serial_output;
+    ESP8266WebServer *_server;
+    char * _username;
+    char * _password;
+    bool _authenticated;
+    String _updaterError;
 };