implement '-e/--error-file' command-line option (#4732)

copying per-URL options from regular, read-only input files
does currently not work

gallery_dl/option.py

@@ -286,6 +286,11 @@ def build_parser():
         help=("Print a list of available keywords and example values "
               "for the given URLs"),
     )
+    output.add_argument(
+        "-e", "--error-file",
+        dest="error_file", metavar="FILE",
+        help="Add input URLs which returned an error to FILE",
+    )
     output.add_argument(
         "--list-modules",
         dest="list_modules", action="store_true",