Unknown but valid chunks should raise WARNING, not major or minor ERRORs

[ronaldtse]

Reasoning: metanorma/pngcheck-ruby#17 (comment)

Approach as discussed with @maxirmx :

Based on your analisys I will patch pngcheck so that it will return STATUS_WARNING if a chunk of unknown type is found

Agree. I know this is not ideal but it's what is possible.

In case of WARNING we have two options:

• ignore it and return true
• raise exception but add status code field to it so that it is possible to recover warnings upstream

The latter please.

[maxirmx]

There is obvious bug in the code.
When a chunk of unknown typw is found error is raised if (PUBLIC(chunkid))
However, the standard says that An unknown chunk type is not treated as an error unless it is a critical chunk, so the condition shall be if (PUBLIC(chunkid) && CRITICAL(chunkid))

[ronaldtse]

Thanks for merging #3 !