common: env_sf: Add exclamation mark

Add exclamation mark to the errmsg, when error and set_default_env.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
Cc: Vignesh R <vigneshr@ti.com>
Cc: Jagan Teki <jteki@openedev.com>
Cc: Ravi Babu <ravibabu@ti.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>