initramfs: add missing decompressor error check

The decompressors return error by calling a supplied error function, and/or
by returning an error return value.  The initramfs code, however, fails to
check the exit code returned by the decompressor, and only checks the error
status set by calling the error function. 
This patch adds a return code check and calls the error function. 
Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
LKML-Reference: <4b26b1ef.0+ZWxT6886olqcSc%phillip@lougher.demon.co.uk>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>