The argument would be that you can't look to the contract because the contract doesn't say what happens if the cherries weigh 4,500 pounds. Everything written in the contract is based on the assumption that the cherries weigh 5,000 pounds. (Unless it contains some clause about the weight, of course.) Here, it is clearly unjust to enforce the contract as agreed because the agreement was predicated on the shared belief.
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.
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?