[BP] Fix write after response commit on proxy response (#7353)

When the Links table is empty an 500 error happens:
java.lang.IllegalStateException: Cannot call sendError() after the response has been committed

This is caused because the response.sendError(...) method is being called twice when
the Links table has no entries. This commits fix this error calling the method only
once.
In addition, it removes unnecessary info returned to the client.

Signed-off-by: Juan Luis Rodríguez <juanluisrp@gmail.com>

web/src/main/java/org/fao/geonet/proxy/URITemplateProxyServlet.java

@@ -362,6 +362,7 @@ protected void service(HttpServletRequest servletRequest, HttpServletResponse se
                 } catch (SecurityException securityException) {
                     servletResponse.sendError(HttpServletResponse.SC_FORBIDDEN,
                         securityException.getMessage());
+                    return;
                 }
 
                 // Check if the link requested is in database link list
@@ -375,19 +376,16 @@ protected void service(HttpServletRequest servletRequest, HttpServletResponse se
                             LinkSpecs.filter(host, null, null,
                                 null, null, null));
                         if (linksFound == 0) {
-                            String message = String.format(
-                                "The proxy does not allow to access '%s' " +
-                                    "because the URL host was not registered in any metadata records.",
-                                uri
-                            );
+                            String message = "The proxy does not allow to access the requested URI " +
+                                "because the URL host was not registered in any metadata records.";
                             if (linkRepository.count() == 0) {
-                                servletResponse.sendError(HttpServletResponse.SC_FORBIDDEN,
-                                    "The proxy is configured with DB_LINK_CHECK mode " +
-                                        "but the MetadataLink table is empty. " +
-                                        "Administrator may need to analyze record links from the admin console " +
-                                        "in order to register URL allowed by the proxy. " + message);
+                                message = "The proxy is configured with DB_LINK_CHECK mode " +
+                                    "but the MetadataLink table is empty. " +
+                                    "Administrator may need to analyze record links from the admin console " +
+                                    "in order to register URL allowed by the proxy.";
                             }
                             servletResponse.sendError(HttpServletResponse.SC_FORBIDDEN, message);
+                            return;
                         }
                         proxyCallAllowed = linksFound > 0;
                     } catch (URISyntaxException e) {