add unavailable media state for plex media (#754)

* rework plex movie library scanner; add unavailable media item state

* plex television scanner improvements

* reset plex etags as needed

* update changelog

ErsatzTV.Core/Errors/ScanCanceled.cs

@@ -0,0 +1,8 @@
+namespace ErsatzTV.Core.Errors;
+
+public class ScanCanceled : BaseError
+{
+    public ScanCanceled() : base("Scan was canceled")
+    {
+    }
+}