mgmt: mcumgr: Make SMP version 2 to legacy error lookup extensible

Replaces the manual lookup function with a lookup function which
is provided when registering MCUmgr handlers which can be used to
find the function to translate error codes, allowing out of tree
MCUmgr handlers to provide error translation handlers.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>