Complete TODO: Add descriptive error messages to docset initialization failure

[amini-allight]

While authoring a new Zeal docset recently I configured the docset wrong and had to debug the issue, but the only error message Zeal would give me was:

Could not load docset from 'PATH'. Reinstall the docset.

I went through the code trying to find the root cause of this error and discovered a TODO in src/libs/registry/docset.cpp suggesting that the errors leading to that message should be reported in more detail. I decided to try fix this myself, which is what this pull request does, adding new error messages for missing directories or files and for a malformed Info.plist.