compat errors should clearly state where each constraint comes from

[aplavin]

Whenever one gets a message like this

empty intersection between [email protected] and project compatibility 2.3.4-2

it's clear where the latter constraint comes from ("project compatibility", presumably [compat] in Project.toml). But what about the first one? One can only guess with experience that it refers to Manifest.toml, but would be much clearer to explicitly state that.

There are other compat errors that could use this improvement as well, I'll try to keep this thread updated whenever I encounter one.