Add a TaggedVersionNotFound error

Resolves #159: Unclear error message when a repository has no tags.

```
$ carthage update
*** Fetching MDPSplitView
No tagged versions found for github "mdiep/MDPSplitView"
```
AFAIK this should work:

```swift
enum ErrorCode: Int {
	…
	case TaggedVersionNotFound = 6
	…
}