If the file for the cover doesn't exist, fetch again

Recovers from the issue highlighted in the previous commit.

May need debounce...

README.md

@@ -37,6 +37,7 @@ Doing so isn't fatal (it's not a secret), but it is annoying for other contribut
     * Top tracks for an artist can now be displayed
     * Similar tracks for an artist (sometimes called "radio") can now be displayed
 * Directories can be starred.
+* Recover from situations where the cover file was deleted from the filesystem.
 * Fix covers being deleted by the system on macOS 15.
 * Fix searches being ran twice.
 * Fix albums with the same ID across multiple servers being mixed.

Submariner/SBSubsonicParsingOperation.swift

@@ -324,7 +324,8 @@ class SBSubsonicParsingOperation: SBOperation, XMLParserDelegate {
                     album!.cover = cover
                 }
 
-                if album?.cover?.imagePath == nil {
+                let imagePath = album?.cover?.imagePath
+                if imagePath == nil || !FileManager.default.fileExists(atPath: imagePath! as String) {
                     server.getCover(id: coverArt, for: album!.itemId)
                 }
             }
@@ -970,7 +971,8 @@ class SBSubsonicParsingOperation: SBOperation, XMLParserDelegate {
                 attachedAlbum.cover = cover
             }
 
-            if attachedAlbum.cover?.imagePath == nil {
+            let imagePath = attachedAlbum.cover?.imagePath
+            if imagePath == nil || !FileManager.default.fileExists(atPath: imagePath! as String) {
                 server.getCover(id: coverArt, for: attributeDict["albumId"])
             }
         }