feat: use custom error servlet to return json rather than default html

WebContent/WEB-INF/web.xml

@@ -5,17 +5,29 @@
       <welcome-file>index.html</welcome-file>
    </welcome-file-list>
 
+   <error-page>
+     <error-code>400</error-code>
+     <location>/error/json</location>
+   </error-page>
+   <error-page>
+     <error-code>403</error-code>
+     <location>/error/json</location>
+   </error-page>
    <error-page>
      <error-code>404</error-code>
-     <location>/error/404error.html</location>
+     <location>/error/json</location>
+   </error-page>
+   <error-page>
+     <error-code>416</error-code>
+     <location>/error/json</location>
    </error-page>
    <error-page>
       <error-code>500</error-code>
-      <location>/error/error.html</location>
+      <location>/error/json</location>
    </error-page>
    <error-page>
       <exception-type>java.lang.Throwable</exception-type>
-      <location>/error/error.html</location>
+      <location>/error/json</location>
    </error-page>
 
    <!-- From Tomcat -->
@@ -574,6 +586,15 @@
    -->
 
    <!-- servlets -->
+   <!-- JSON Error Handler Servlet -->
+   <servlet>
+      <servlet-name>JsonErrorServlet</servlet-name>
+      <servlet-class>prerna.web.conf.JsonErrorServlet</servlet-class>
+   </servlet>
+   <servlet-mapping>
+      <servlet-name>JsonErrorServlet</servlet-name>
+      <url-pattern>/error/json</url-pattern>
+   </servlet-mapping>
    <!-- Servlet For SEMOSS UI - SEMOSS BE communication -->
    <servlet>
       <servlet-name>MainApplication</servlet-name>

src/prerna/web/conf/JsonErrorServlet.java

@@ -0,0 +1,60 @@
+package prerna.web.conf;
+
+import java.io.IOException;
+
+import javax.servlet.http.HttpServlet;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+
+import com.google.gson.Gson;
+
+import java.util.LinkedHashMap;
+import java.util.Map;
+
+/**
+ * Servlet that handles Tomcat error pages and returns JSON responses
+ * instead of the default HTML error pages.
+ */
+public class JsonErrorServlet extends HttpServlet {
+
+	private static final long serialVersionUID = 1L;
+	private static final Gson GSON = new Gson();
+
+	@Override
+	protected void service(HttpServletRequest request, HttpServletResponse response) throws IOException {
+		// Tomcat sets these request attributes when forwarding to error pages
+		Integer statusCode = (Integer) request.getAttribute("javax.servlet.error.status_code");
+		String message = (String) request.getAttribute("javax.servlet.error.message");
+		String requestUri = (String) request.getAttribute("javax.servlet.error.request_uri");
+
+		if (statusCode == null) {
+			statusCode = HttpServletResponse.SC_INTERNAL_SERVER_ERROR;
+		}
+		if (message == null || message.isEmpty()) {
+			message = getDefaultMessage(statusCode);
+		}
+
+		Map<String, Object> errorBody = new LinkedHashMap<>();
+		errorBody.put("status", statusCode);
+		errorBody.put("message", message);
+		if (requestUri != null) {
+			errorBody.put("request_uri", requestUri);
+		}
+
+		response.setStatus(statusCode);
+		response.setContentType("application/json");
+		response.setCharacterEncoding("UTF-8");
+		response.getWriter().write(GSON.toJson(errorBody));
+	}
+
+	private static String getDefaultMessage(int statusCode) {
+		switch (statusCode) {
+			case 400: return "Bad Request";
+			case 403: return "Forbidden";
+			case 404: return "Not Found";
+			case 416: return "Requested Range Not Satisfiable";
+			case 500: return "Internal Server Error";
+			default:  return "Error";
+		}
+	}
+}