Cope with repeated additions

[andymckay]

If you are adding an issue to a project board, it will do so when you label it. Yay.

Finished running: Successfully created a new card in column #8215313 for an issue with the corresponding id:629435434 !

Then the next time you add a label it will try to add it again, because the issue still has the label. At this point the Action fails and the build fails.

Attempting to create a card in column 8215313, for an issue with the corresponding id #629435434 #ERROR# HttpError: Validation Failed

It would be super helpful if it checked if the card exists on the column before it tries to add and if it does, just succeeds and continues on with the next part of the build.

[ritchxu]

Check out #14