fix: catch exception when getting notes folder

[enjeck]

Currently, the app crashes the server with the following error. This should fix it.

Internal Server Error
The server was unable to complete your request.

If this happens again, please send the technical details below to the server administrator.

More details can be found in the server log.

Technical details
Remote Address: 192.168.21.4
Request ID: 1b5EZixWS2bZK7VIVCVm
Type: OCA\Notes\Service\NotesFolderException
Code: 0
Message: Notes is not a folder
File: /var/www/html/apps/notes/lib/Service/NoteUtil.php
Line: 213

Trace
#0 /var/www/html/apps/notes/lib/Service/NoteUtil.php(189): OCA\Notes\Service\NoteUtil->getOrCreateNotesFolder('admin', false)
#1 /var/www/html/apps/notes/lib/AppInfo/Capabilities.php(29): OCA\Notes\Service\NoteUtil->getNotesFolderUserPath('admin')
#2 /var/www/html/lib/private/CapabilitiesManager.php(61): OCA\Notes\AppInfo\Capabilities->getCapabilities()
#3 /var/www/html/lib/private/Template/JSConfigHelper.php(137): OC\CapabilitiesManager->getCapabilities(false, true)
#4 /var/www/html/lib/private/TemplateLayout.php(239): OC\Template\JSConfigHelper->getConfig()
#5 /var/www/html/lib/private/legacy/OC_Template.php(120): OC\TemplateLayout->__construct('user', 'dashboard')
#6 /var/www/html/lib/public/AppFramework/Http/TemplateResponse.php(189): OC_Template->fetchPage(Array)
#7 /var/www/html/lib/private/AppFramework/Http/Dispatcher.php(159): OCP\AppFramework\Http\TemplateResponse->render()
#8 /var/www/html/lib/private/AppFramework/App.php(161): OC\AppFramework\Http\Dispatcher->dispatch(Object(OCA\Dashboard\Controller\DashboardController), 'index')
#9 /var/www/html/lib/private/Route/Router.php(306): OC\AppFramework\App::main('OCA\\Dashboard\\C...', 'index', Object(OC\AppFramework\DependencyInjection\DIContainer), Array)
#10 /var/www/html/lib/base.php(1018): OC\Route\Router->match('/apps/dashboard...')
#11 /var/www/html/index.php(24): OC::handleRequest()
#12 {main}