Placeholder verification has confusing error messages

[robotlolita]

During the verification phase, if there are any placeholder entities that have not been fulfilled for the package, the package loader will stop with an error. Currently the error looks something like:

Cannot open <package> because it's lacking definitions for: <entity>...

This message lacks any indication of where these entities are used, forcing users to search for them. But it also does not suggest fixes. For example, forgetting to open a package's namespace and using types from that package would result in that message, but the fix would be to open the package's namespace, not to add a new local definition. Typos, similarly, should be reported as possible typos, not as a lacking definition.