Replace most occurrences of os.Exit(0) in the backend and sync packages with a custom backend.Exit function that can allow "soft exit" in GUI/TUI environments to avoid killing the entire process; currently all major errors still result in killing the entire process

src/backend/edit.go

@@ -37,7 +37,7 @@ func Rename(oldLocation, newLocation string) {
 	}
 
 	// TODO implement synced renaming
-	os.Exit(0)
+	Exit(0)
 }
 
 // EnsureSliceLength ensures slice is long enough to contain the specified index