Improve error message when importing an unusable package

If a module cannot be found because it is ignored or from an unusable
package, report this to the user and the reason it is unusable.

Currently, GHC displays the standard "Cannot find module error". For
example:

```
<no location info>: error:
    Could not find module ‘Control.Monad.Random’
    Perhaps you meant
      Control.Monad.Reader (from mtl-2.2.2)
      Control.Monad.Cont (from mtl-2.2.2)
      Control.Monad.Error (from mtl-2.2.2)
```

GHC does, however, indicate unusable/ignored packages with the -v flag:

```
package MonadRandom-0.5.1-1421RgpXdhC8e8UI7D3emA is unusable due to
missing dependencies:
  fail-4.9.0.0-BAHmj60kS5K7NVhhKpm9J5
```

With this change, I took that message and added it to the output of the
"Cannot find module" message.

Reviewers: bgamari, dfeuer

Reviewed By: bgamari

Subscribers: Phyx, dfeuer, rwbarton, thomie, carter

GHC Trac Issues: #4806

Differential Revision: https://phabricator.haskell.org/D4783