Fix corner case when pinning to a not available version (#767)

Fix corner case when pinning to a not available version (#767)

* Less confusing user feedback for existing pin

* Typo in comment

* Fix for corner case in which no solution was found

The solution is impossible, but still an incomplete solution should be
returned. In essence, when the user requested an impossible situation by
applying a pin to a non-existent version for a crate already in the
dependencies, the solver reused the old version assuming it would succeed,
which was not the case if the pin is impossible.

* Better pin info separation

* Extra test case for pinning to unavailable version