dns: use the CHERIOT_DURING macros instead of on_error

Similar to the similar change we did in the TCP/IP compartment, this
reduces code size by 184B. Replacing the two `on_error` with macros
makes us gain 128B, and we gain another 56B by optimizing error paths.

Signed-off-by: Hugo Lefeuvre <[email protected]>

Author: hlef