musrview_check: report missing Pillow/numpy explicitly

If Pillow or numpy aren't importable, fail fast with a clear
**ERROR** message and pip-install hint instead of a bare traceback
from inside pixel_diff().

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/musrview_check/musrview_check.py

@@ -25,6 +25,22 @@ import tempfile
 import time
 
 
+def check_python_deps():
+    """Make sure Pillow and numpy are importable; print a clear, actionable
+    error message (rather than a bare traceback) if they are not."""
+    missing = []
+    for module, pip_name in (("PIL", "Pillow"), ("numpy", "numpy")):
+        try:
+            __import__(module)
+        except ImportError:
+            missing.append(pip_name)
+    if missing:
+        print(f"**ERROR** missing required python package(s): {', '.join(missing)}")
+        print(f"  install with: {sys.executable} -m pip install {' '.join(missing)}")
+        return False
+    return True
+
+
 def pixel_diff(img_a_path, img_b_path):
     """Return the mean absolute pixel difference normalised to [0, 1]."""
     from PIL import Image
@@ -54,6 +70,9 @@ def main():
     # everything after the known args is forwarded to musrview
     args, musrview_opts = parser.parse_known_args()
 
+    if not args.generate and not check_python_deps():
+        return 1
+
     msr_basename = os.path.splitext(os.path.basename(args.msr_file))[0]
     ref_subdir = os.path.join(args.ref_dir, args.test_name)