More descriptive error message for bundle upload errors

[jamesbeedy]

I didn't have a README.md in my bundle dir, when trying to upload to the store, I get an unhelpful error when doesn't exist.

http://paste.ubuntu.com/23996456/

Need more descriptive user facing message here.

[Brad Crittenden]

Thanks for reporting the bug @jamesbeedy. Just to be clear, the error message said it was missing metadata.yaml when it was really only missing the README? Or are you commenting on the unfriendly message (which I agree should be fixed)?

[Jay R. Wren]

I think this is a dupe of #106