handle misspelled and removed packages

[Xiphe]

Not sure if I got that right but I assume the following:

---

Given I have a list of previouslyUnreleased packages
And I remove a package from source without removing it from previouslyUnreleased
Then I expect rlsr to ignore that package or give me a warning

But the following code with create a tag undefined@undefined or even fail beforehand.

https://github.com/matthias-reis/rlsr/blob/master/src/runners/perform.js#L23-L30