Clearer messages when the conflict is caused by strict priority configurations

[jaimergp]

Checklist

• I added a descriptive title
• I searched open requests and couldn't find a duplicate

What is the idea?

When the solver cannot find a solution with strict priority enabled, we say PackagesNotFoundError, because the libsolv message is:

Encountered problems while solving:
  - nothing provides requested <package-name>

At that point, libmamba might have thrown several warnings like

warning  libmamba Problem type not implemented SOLVER_RULE_STRICT_REPO_PRIORIT

Which is a hint for some experienced users, but it's not super obvious, and not close to the final message (which is often the only one checked).

Why is this needed?

Better UX when the solver fails to find a solution.

What should happen?

We could decorate the PackagesNotFoundError message with a little hint like "It looks like you have channel_priority set to strict, which can sometimes cause this exception. Consider using flexible for this command using ...".

Additional Context

Comes from https://matrix.to/#/!SOyumkgPRWoXfQYIFH:matrix.org/$170049404134SekiW:gitter.im?via=matrix.org&via=gitter.im&via=staffchat.ethz.ch