erofs: improve LZ4 error strings

erofs: improve LZ4 error strings

Just like what was done for other algorithms, let's propagate detailed
error reasons for LZ4 instead of just -EFSCORRUPTED to users:

 "corrupted compressed data":
    the compressed data is malformed or
      destination buffer is not large enough

 "unexpected end of stream":
    the compressed stream ends normally, but without producing enough
    decompressed data.

 "compressed data start not found":
    can be returned by z_erofs_fixup_insize().

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>