tools: fw_env: Fix warning when reading too little

When using CONFIG_ENV_IS_IN_FAT and the config-file specifies a size
larger than what U-Boot wrote into the env-file, a confusing error
message is shown:

    $ fw_printenv
    Read error on /boot/uboot.env: Success

Fix this by showing a different error message when read returns too
little data.

Signed-off-by: Harald Seiler <hws@denx.de>