Fixed the behaviour for an update without mirror

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Related tickets | [GC#18](https://code.google.com/p/cyg-apt/issues/detail?id=18)
| License       | GNU GPL v3

@nylen Error messages are there explicit enough?