Better deal with no-longer-existing ports

Disconnected ports could still happen Even when the port
was still seen as valid internally. Using a try-catch
block makes invalid port detection more reliable. This
is an occurrence I often encountered when stepping into
content script code, causing suprious error messages to
be thrown into uBO's background dev console.

platform/common/vapi-background.js

@@ -1086,11 +1086,13 @@ vAPI.messaging = {
         }
         proxy(response) {
             // https://github.com/chrisaljoudi/uBlock/issues/383
-            if ( this.messaging.ports.has(this.port.name) ) {
+            try {
                 this.port.postMessage({
                     msgId: this.msgId,
                     msg: response !== undefined ? response : null,
                 });
+            } catch (ex) {
+                this.messaging.onPortDisconnect(this.port);
             }
             // Store for reuse
             this.port = null;