"--read-only gives weird error on update".
It is not fixable in 4.0 because it requires modifying
the sql/share/*/errmsg.txt files. So it is fixed in 4.1 like this:
the ER_SKIP_GRANT_TABLES is replaced by a more generic
ER_OPTION_PREVENTS_STATEMENT which can be used both for
"can't do this because of --skip-grant-tables" and
"can't do this because of --read-only" (for this we don't use
ER_CANT_UPDATE_WITH_READLOCK anymore).
So now the message for --read-only is:
"The MySQL server is running with the --read-only option so
cannot execute this statement".