env: add failing trace in env_erase

Add trace in env save to indicate any errors to end user and avoid
silent output when the command 'env erase' is not executed as it is
done in env_save with commit 8968288c ("env: add failing trace in
env_save")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Heiko Schocher <hs@denx.de>