Replace RuntimeException with IOException

The RuntimeException was not explicitly declared and thus not caught at every call of this constructor. This change ensures that this possible exception is handled by the dedicated error handlers.

app/src/main/java/org/schabi/newpipe/streams/io/StoredFileHelper.java

@@ -130,7 +130,7 @@ public class StoredFileHelper implements Serializable {
             final DocumentFile file = DocumentFile.fromSingleUri(context, path);
 
             if (file == null) {
-                throw new RuntimeException("SAF not available");
+                throw new IOException("SAF not available");
             }
 
             this.context = context;