This allows us to reliably parse the error in code, rather than
attempt to parse the error text. Because the error text gets
translated into the user's language, this error-handling path
wasn't functioning at all for users using Zulip in any of the
seven non-English languages for which we had a translation for
this string.
Together with 709c3b50f which fixed a similar issue in a
different error-handling path, this fixes#5598.