make fallback error dismissable

sometimes, an error is not that fatal, and the user may want
to dismiss it, so add a button to do exactly that

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

app/error-handler.js

@@ -17,9 +17,11 @@ function handleError(event, err) {
             return false;
         }
 
-        // Only show the initial error
+        // Clear last error
         if (msg.hasChildNodes()) {
-            return false;
+            while (msg.firstChild) {
+                msg.removeChild(msg.firstChild);
+            }
         }
 
         let div = document.createElement("div");
@@ -58,5 +60,14 @@ function handleError(event, err) {
     return false;
 }
 
+function hideError() {
+    document.getElementById('noVNC_fallback_error')
+        .classList.remove("noVNC_open");
+}
+
+window.addEventListener('load', () => {
+    document.getElementById("noVNC_error_dismiss_button")
+        .addEventListener('click', hideError);
+});
 window.addEventListener('error', evt => handleError(evt, evt.error));
 window.addEventListener('unhandledrejection', evt => handleError(evt.reason, evt.reason));

vnc.html

@@ -66,6 +66,7 @@
             <div>noVNC encountered an error:</div>
             <br>
             <div id="noVNC_fallback_errormsg"></div>
+            <input id="noVNC_error_dismiss_button" type="button" value="Dismiss" class="noVNC_submit">
         </div>
     </div>