For unsatisfied request we should be able to print some diagnostic message why
When updater can't satisfy some request it is almost cryptic work to find out why. For development this should be solved when #147 is implemented (it should be visible from dependency graph), but we should also create way for user to found out why something can't be installed. So maybe some diagnostic command that would just print reasons why some package in some version can't be installed. Or/And print diagnostic message when request isn't satisfied.