Returns an error if the condition is not met

[zhouhao3]

When some of the conditions do not meet, you can not only return nil, but also return an error message to terminate the program.
Signed-off-by: zhouhao zhouhao@cn.fujitsu.com

[wking]

On Thu, Dec 08, 2016 at 10:34:33PM -0800, Zhou Hao wrote: When some of the conditions do not meet, you can not only return nil, but also return an error message to terminate the program.

This PR is very similar to #83, and I recommend closing this as well (although there's still room for improving the UI [1]). Having no references is *not* an error. It is surprising, hence the warning, but an image without refs is technically legal and has some legitimate use cases. More background in opencontainers/image-spec#459, opencontainers/image-spec#287, and the #83 discussion. On the other hand, finding a subdirectory under `refs/` *may* be an error. The current spec wording has a few instances of “the refs subdirectories”, including the charset restriction [2]. But my guess is that the intention was to restrict refs to a single directory. I'm not sure if the intention for refs/somedir/ was to ignore it or error out though. An image-spec PR to clarify this point is probably in order. [1]: #83 (comment) [2]: https://github.com/opencontainers/image-spec/blob/v1.0.0-rc3/image-layout.md

[zhouhao3]

Well, it really should not be so modified here, but I always feel that this place to determine some problems.