pytest plugin assert_error

[lengau]

What needs to get done

Like the various message asserts, the pytest plugin's emitter should be able to assert that the error method was called and check the corresponding craft error.

Why it needs to get done

Right now there's no way to check that the correct error was sent to emit.error

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3686.

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[lengau]

This is getting reverted in #311.

The feature works, but I didn't catch when creating it that it's a breaking change, so we'll need to assign it to the next major release.