Propagate Error Status to the Shell on fw_printenv Errors

Changed implementation such that fw_printenv returns failure status
when one or more specified variables do not exist or when incorrect
command syntax is used.

This aids scripting fw_printenv such that the script can key of the
return status rather than relying on standard error "scraping".

Signed-off-by: Grant Erickson <gerickson@nuovations.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>