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"
```
We could also give `ErrorCode` a method to make this more convenient:

```swift
func error(userInfo: [NSObject: AnyObject]?) -> NSError {
	return NSError(domain: CarthageErrorDomain, code: self, userInfo: userInfo)
}
```