fix error-handling in worker, tweak example (#288)

Author: bryangingechen

examples/GUI/gui.js

@@ -32,6 +32,10 @@ function execute(commands) {
 	worker.onmessage = function (event) {
 		var results = event.data.results;
 		toc("Executing SQL");
+		if (!results) {
+			error({message: event.data.error});
+			return;
+		}
 
 		tic();
 		outputElm.innerHTML = "";

src/worker.coffee

@@ -48,4 +48,8 @@ if typeof importScripts is 'function' # Detect webworker context
                 when 'close'
                     db?.close()
                 else
-                    throw new 'Invalid action : ' + data?['action']
+                    throw new Error 'Invalid action : ' + data?['action']
+        .catch (err) ->
+            postMessage
+                'id': event['data']['id']
+                'error': err['message']