add a comment on parsing file not found error.

src/cal_tools/tools.py

@@ -358,6 +358,8 @@ def get_dir_creation_date(directory: Union[str, Path], run: int,
     proposal = int(directory.parent.name[1:])
     directory = directory / 'r{:04d}'.format(run)
 
+    # Validate the availability of the input folder.
+    # And show a clear error message, if it was not found.
     try:
         dc = RunDirectory(directory)
     except FileNotFoundError as e: