Fix verapdf NotADirectoryError crash on some platforms

Catch OSError (parent of both FileNotFoundError and
NotADirectoryError) in verapdf.available() so environments where
executing `verapdf` raises NotADirectoryError gracefully fall back
instead of crashing the pipeline. Fixes #1638.

src/ocrmypdf/_exec/verapdf.py

@@ -36,7 +36,7 @@ def available() -> bool:
     """Check if verapdf is available."""
     try:
         version()
-    except MissingDependencyError:
+    except (MissingDependencyError, OSError):
         return False
     return True