fix: report un-optimizable images as a warning, not a traceback

The optimizer is best-effort: any image it cannot process is left
unchanged in the output, which remains valid. Previously, an extraction
failure (e.g. an exotic colorspace pikepdf cannot transcode) was logged
with log.exception, printing a full traceback at ERROR level that alarmed
users even though nothing was wrong with the output (issue #846).

Trap such failures with a concise warning that the image was left
unchanged and the output is still valid, and demote the traceback to
debug verbosity for diagnosis.

src/ocrmypdf/optimize.py

@@ -354,9 +354,16 @@ def extract_images(
                 pdf=pdf, root=root, image=image, xref=xref, options=options
             )
         except Exception:  # pylint: disable=broad-except
-            log.exception(
-                f"xref {xref}: While extracting this image, an error occurred"
+            # Optimization is best-effort: an image we cannot process is simply
+            # left unchanged in the output, which remains valid. Report this as
+            # a concise warning rather than an alarming traceback (issue #846);
+            # the full detail is still available at debug verbosity.
+            log.warning(
+                f"xref {xref}: this image could not be processed by the "
+                "optimizer and was left unchanged. The output file is still "
+                "valid."
             )
+            log.debug(f"xref {xref}: image optimization error detail", exc_info=True)
             errors += 1
         else:
             if result: