Add unhandledrejection handling

src/error-handling-basic.js

@@ -17,3 +17,7 @@ window.onerror = function (msg, url, lineNo, columnNo, error) {
 	}
 	return false;
 };
+
+window.onunhandledrejection = function (event) {
+	alert('Unhandled Rejection: ' + event.reason);
+}

src/error-handling-enhanced.js

@@ -14,6 +14,10 @@ window.onerror = function (message, source, lineno, colno, error) {
 	}
 };
 
+window.onunhandledrejection = function (event) {
+	show_error_message(localize("Internal application error.") + "\nUnhandled Rejection.", event.reason);
+};
+
 if (/MSIE \d|Trident.*rv:/.test(navigator.userAgent)) {
 	document.write(
 		'<style>body { text-align: center; }</style>' +