Fix multiple error cases reached

With `textual run --dev "src.textual_paint.gallery foobar"` it output:

    Folder not found: foobar
    Not a folder: foobar
    No ANSI art (*.ans, *.txt) found in folder: foobar

    NOTE: 3 errors shown above.

Now it correctly outputs only:

    Folder not found: foobar

src/textual_paint/gallery.py

@@ -99,9 +99,12 @@ class GalleryApp(App[None]):
 
         if not gallery_folder.exists():
             self.exit(None, f"Folder not found: {gallery_folder}")
+            return
 
         if not gallery_folder.is_dir():
+            # TODO: allow showing a specific file, and load whatever folder it's in
             self.exit(None, f"Not a folder: {gallery_folder}")
+            return
 
         exts = (".ans", ".txt")
 
@@ -129,6 +132,7 @@ class GalleryApp(App[None]):
 
         # Debugging
         # self.exit(None, "\n".join(str(path) for path in paths))
+        # return
 
         for path in paths:
             if path.suffix not in exts:
@@ -141,6 +145,7 @@ class GalleryApp(App[None]):
         
         if len(self.scroll.children) == 0:
             self.exit(None, f"No ANSI art ({', '.join(f'*{ext}' for ext in exts)}) found in folder: {gallery_folder}")
+            return
 
     def _scroll_to_adjacent_item(self, delta_index: int = 0) -> None:
         """Scroll to the next/previous item."""