init the awkward code

static/html/jspaint/src/error-handling-enhanced.js

@@ -0,0 +1,52 @@
+// Use only ES5 syntax for this script, as it's meant to handle old IE.
+
+// Note that this can't simply be merged with the other onerror handler with a try/catch,
+// because showMessageBox is async, and could throw an error before dependencies are met (or if there was an error in the error handling),
+// and try doesn't catch errors in async code. It would need to be awaited.
+// And making show_error_message return a promise might cause subtle problems due to the pattern of `return show_error_message()`.
+var old_onerror = window.onerror;
+window.onerror = function (message, source, lineno, colno, error) {
+	try {
+		// Some errors don't give an error object, like "ResizeObserver loop limit exceeded"
+		show_error_message(localize("Internal application error."), error || message);
+	} catch (e) {
+		old_onerror(message, source, lineno, colno, error);
+		console.warn("Error in error handler:", e);
+	}
+};
+
+var old_onunhandledrejection = window.onunhandledrejection;
+var restore_new_onunhandledrejection_tid;
+var new_onunhandledrejection = function (event) {
+	// Just in case show_error_message triggers a new unhandledrejection event,
+	// we need to make sure we don't call it again.
+	// Test by adding to the top of show_error_message:
+	// Promise.reject(new Error("EMIT EMIT EMIT"))
+	// Also test:
+	// throw new Error("EMIT EMIT EMIT");
+	// I want my error handling to be RESILIENT!
+	window.onunhandledrejection = old_onunhandledrejection;
+	clearTimeout(restore_new_onunhandledrejection_tid);
+	restore_new_onunhandledrejection_tid = setTimeout(function () {
+		window.onunhandledrejection = new_onunhandledrejection;
+	}, 0);
+
+	try {
+		show_error_message(localize("Internal application error.") + "\nUnhandled Rejection.", event.reason);
+	} catch (e) {
+		old_onunhandledrejection(event);
+		console.warn("Error in unhandledrejection handler:", e);
+	}
+};
+window.onunhandledrejection = new_onunhandledrejection;
+
+// Show a message for old Internet Explorer.
+if (/MSIE \d|Trident.*rv:/.test(navigator.userAgent)) {
+	document.write(
+		'<style>body { text-align: center; }</style>' +
+		'<div className="not-supported">' +
+		'	<h1 className="not-supported-header">Internet Explorer is not supported!</h1>' +
+		'	<p className="not-supported-details">Try Chrome, Firefox, or Edge.</p>' +
+		'</div>'
+	);
+}