gui, lib/ignore: Handle editing ignores with error (fixes #5425) (#6757)

This changes the error handling in loading ignores slightly:

- There is a new ParseError type that is returned as the error
  (somewhere in the chain) when the problem was not an I/O error loading
  the file, but some issue with the contents.

- If the file was read successfully but not parsed successfully we still
  return the lines read (in addition to nil patterns and a ParseError).

- In the API, if the error IsParseError then we return a successful
  HTTP response with the lines and the actual error included in the JSON
  object.

- In the GUI, as long as the HTTP call to load the ignores was
  successful we can edit the ignores. If there was an error we show this
  as a validation error on the dialog.

Also some cleanup on the Javascript side as it for some reason used
jQuery instead of Angular for this editor...

gui/default/syncthing/folder/editFolderModalView.html

@@ -9,6 +9,7 @@
         <li><a data-toggle="tab" href="#folder-advanced"><span class="fas fa-cogs"></span> <span translate>Advanced</span></a></li>
       </ul>
       <div class="tab-content">
+
         <div id="folder-general" class="tab-pane in active">
           <div class="form-group" ng-class="{'has-error': folderEditor.folderLabel.$invalid && folderEditor.folderLabel.$dirty}">
             <label for="folderLabel"><span translate>Folder Label</span></label>
@@ -43,6 +44,7 @@
             </p>
           </div>
         </div>
+
         <div id="folder-sharing" class="tab-pane">
           <div class="form-group" ng-if="currentFolder.sharedDevices.length">
             <label translate>Currently Shared With Devices</label>
@@ -82,6 +84,7 @@
             </div>
           </div>
         </div>
+
         <div id="folder-versioning" class="tab-pane">
           <div class="form-group">
             <label translate>File Versioning</label>&emsp;<a href="https://docs.syncthing.net/users/versioning.html" target="_blank"><span class="fas fa-question-circle"></span>&nbsp;<span translate>Help</span></a>
@@ -142,9 +145,15 @@
             </p>
           </div>
         </div>
+
         <div id="folder-ignores" class="tab-pane">
           <p translate>Enter ignore patterns, one per line.</p>
-          <textarea class="form-control" rows="5"></textarea>
+          <div ng-class="{'has-error': ignores.error != null}">
+            <textarea class="form-control" rows="5" ng-model="ignores.text" ng-disabled="ignores.disabled"></textarea>
+            <p class="help-block" ng-if="ignores.error">
+              {{ignores.error}}
+            </p>
+          </div>
           <hr />
           <p class="small"><span translate>Quick guide to supported patterns</span> (<a href="https://docs.syncthing.net/users/ignoring.html" target="_blank" translate>full documentation</a>):</p>
           <dl class="dl-horizontal dl-narrow small">
@@ -165,6 +174,7 @@
           <span translate ng-show="editingExisting" translate-value-path="{{currentFolder.path}}{{system.pathSeparator}}.stignore">Editing {%path%}.</span>
           <span translate ng-show="!editingExisting" translate-value-path="{{currentFolder.path}}{{system.pathSeparator}}.stignore">Creating ignore patterns, overwriting an existing file at {%path%}.</span>
         </div>
+
         <div id="folder-advanced" class="tab-pane">
           <div class="row form-group" ng-class="{'has-error': folderEditor.rescanIntervalS.$invalid && folderEditor.rescanIntervalS.$dirty}">
             <div class="col-md-12">