If in fact the police officers had been robbed and the culprit had gotten away, that'd clearly be a failure. But if you try to catch a drug dealer and that drug dealer attempts to rob you and you catch him, you have now caught someone who not only deals drugs but robs their customers - seems like a success to me!

The perp was arrested for "possession with intent to sell". In other words, it seems like he had drugs and was offering to sell them, but in this particular instance he chose to instead attempt theft. The only indication I see of a possible error is that the officers put the confidential informant in danger -- that person presumably didn't sign up to get attacked.

(And obviously there are corrupt officers who make stuff up, so it's always possible that the perp didn't actually have drugs and they were planted on him, etc. etc. But I'm taking the article at face value for the sake of argument.)