missing dependencies throw assertion errors instead of dependency solving error

for example `legacy`

``` python
PS C:\Users\itziakos\Projects\haas> jaguar --no-debug install ipython-3.0.0-1
Fetching package repositories indices.. done
One of the requested package has broken dependencies
(Dependency solving error: Could not resolve "terminado 0.5" required by "RemotePackageMetadata('ipython-3.0.0-1', key='ipython-3.0.0-1.egg', available=True, product='free', repository_info='BroodRepository(, )')")
```

while `sat_solver`

``` python
PS C:\Users\itziakos\Projects\haas> jaguar --no-debug install ipython-3.0.0-1
Fetching package repositories indices.. done
Unexpected error
AssertionError("No candidates found for requirement 'terminado', needed for dependency RepositoryPackageMetadata('ipython-3.0.0-1', repo=Repository())",)
```