Improve user-facing warnings about invalid ODD in visual editor
When hitting "Save" in the Visual ODD Editor with Dodis-New.odd loaded, the save operation silently fails, with the following error raised in the console.
mixins.js:32 Uncaught TypeError: Cannot read property 'replace' of null at Object.Mixin.escape (mixins.js:32) at Object.<anonymous> (resources/scripts/tags/parameter.tag.js:25) at mixins.js:61 at mixins.js:49 at Array.forEach (<anonymous>) at Object.Mixin.forEachTag (mixins.js:48) at Object.Mixin.serializeTag (mixins.js:60) at Object.<anonymous> (resources/scripts/tags/model.tag.js:211) at mixins.js:61 at mixins.js:49
Tested on multiple machines running tei-publisher-app v3.1.0 and current versions of Chrome and Firefox on macOS 10.13.5 and Ubuntu 17.10.
The active URL when the error appears (assuming the ODD has already been uploaded to TEI Publisher App) is: http://localhost:8080/exist/apps/tei-publisher/odd-editor.html?root=%2Fdb%2Fapps%2Ftei-publisher%2Fodd&outputRoot=%2Fdb%2Fapps%2Ftei-publisher%2Ftransform&odd=Dodis-New.odd&outputPrefix=transform.