fix: surface mask-validation ValueError as gr.Error in run_pipeline

Critique of 89dfe5f caught an inconsistency: on_preview_crop wraps
_compute_crop_and_mask in try/except so the ValueError messages from
mask_to_bbox / compute_crop_region (e.g. "The drawn area is too small",
"Watermark area is too large", "Frame is too small") show up as clean
status text. run_pipeline did NOT wrap the same call, so clicking
Remove Watermark with a too-small mask surfaced as a raw ValueError
stack trace in the Gradio UI instead of a styled red toast.

Now run_pipeline catches ValueError from _compute_crop_and_mask and
re-raises as gr.Error, matching the on_preview_crop UX.

Also tidy the upload-status notes formatting: when both auto-trim and
fps-cap apply (a >15s 60fps clip), the two warnings are now separated
by a blank line so they don't visually run together.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app.py

@@ -299,7 +299,9 @@ def on_video_upload(video_path: str | None):
                 f"⚠️ Source is {meta.fps:.0f} fps — output will be "
                 f"{PROCESS_FPS_MAX:.0f} fps to fit GPU budget."
             )
-        notes_str = ("\n\n" + "\n".join(notes)) if notes else ""
+        # Double newline between notes so they don't visually run together
+        # in the Gradio status box.
+        notes_str = ("\n\n" + "\n\n".join(notes)) if notes else ""
         return (
             gr.update(value=editor_val),
             gr.update(value=None),
@@ -498,9 +500,16 @@ def run_pipeline(
         raise gr.Error("Draw over the watermark before processing.")
 
     progress(0.05, desc="Computing crop region…")
-    meta, _bbox, crop_region, inpaint_mask = _compute_crop_and_mask(
-        raw_mask, meta_state, context_px,
-    )
+    try:
+        meta, _bbox, crop_region, inpaint_mask = _compute_crop_and_mask(
+            raw_mask, meta_state, context_px,
+        )
+    except ValueError as e:
+        # mask_to_bbox / compute_crop_region raise ValueError with user-
+        # facing messages ("drawn area too small", "watermark too large",
+        # "frame too small", etc.). Surface as gr.Error so the UI shows a
+        # clean red toast instead of a generic stack-traced exception.
+        raise gr.Error(str(e)) from e
 
     # Cap working fps so per-frame LaMa and per-chunk VACE both fit within
     # their @spaces.GPU duration budgets at the worst-case input rate.