fwu: print a message if empty capsule checks fail

fwu: print a message if empty capsule checks fail

When dealing with processing of the empty capsule, the capsule gets
applied only when the checks for the empty capsule pass. Print a
message to highlight if empty capsule checks fail, and return an error
value, similar to the normal capsules.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>