That's a bad analogy. Patrick Harnett had full control over who he lent to in order to reduce correlated risk. He had information on what exactly applicants claimed to be using the money for and the ability to demand as much evidence of this as necessary. Based on this evidence, he falsely assumed that his borrowers weren't exposed to Pirate and got screwed - that's the incorrect belief that's the problem here, and the people who loaned Patrick money didn't have this information! They had to rely on Patrick's promise that he was competent to vet applicants and that he'd made sure not to lend money to people who'd just invest it in Pirate.
First, your last sentence is factually untrue. They did not have to rely on Patrick's promise. They could have asked for any evidence they wanted. Second, it wasn't a "promise". It was Patrick's statement of his belief. Lastly, this smacks of the same "due diligence" argument rightly ridiculed regularly in this forum. You can't fault someone for not doing the impossible.
A closer analogy would be if one party entered into a contract in which they gave another party money which the second party was to buy a lorry-load of cherries with, and they'd split the profit from reselling them. If the second party then goes and buys off the back of a truck in some parking lot and gets crates full of rubble instead, which party should be liable?
Exactly. In this case, the loans that formed the basis of the contract turned out not to be what both sides thought they were through a failure shared equally by both sides.