If set, indicates that an error occurred. The meaning of this
value depends on the operation being performed. Generally, it is
a short tag. For example,
OOM to indicate "out of
memory". Before presenting the tag to a user, it must be
translated to a human-readable string. The strings in member
args provide additional info about why the error occurred.
- "result": "NOT_NEWER",
- "error": "Error message (present if an error occurred)."