"Failed" error message from env_load() only clutters the log with
unnecessary details, as we already have all needed warnings by that
time. Example:

    Loading Environment from FAT... MMC: no card present
    ** Bad device mmc 0 **
    Failed (-5)

Let's only print it in case when DEBUG is defined to keep log clear.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>