Every ~4th transaction generates bad JSON

[alexlmiller]

Creating and signing the test transactions when fine, but when trying to broadcast the first time, got this in the cli:

 alex@2021-MBP-Hiro multisig-cli % npm start -- broadcast --json-txs txs/ltt-1-test-full.json --out-file txs/ltt-1-test-broadcast.json
> start
> npm run cli -- broadcast --json-txs txs/ltt-1-test-full.json --out-file txs/ltt-1-test-broadcast.json
> cli
> ts-node ./src/index.ts broadcast --json-txs txs/ltt-1-test-full.json --out-file txs/ltt-1-test-broadcast.json
/Users/alex/Documents/GitHub/multisig-cli/node_modules/@stacks/transactions/src/builders.ts:548
      throw Error(`Failed to broadcast transaction: ${(e as Error).message}`);
            ^
Error: Failed to broadcast transaction: Unexpected token 'F', "Failed to "... is not valid JSON
    at broadcastRawTransaction (/Users/alex/Documents/GitHub/multisig-cli/node_modules/@stacks/transactions/src/builders.ts:548:13)
    at processTicksAndRejections (node:internal/process/task_queues:95:5)

I opened up ltt-1-test-full.json and removed the one line of JSON that looked different than the others (both the original file and the one where I removed the line are included), ran it again and it successfully broadcast (though with one transaction failing due to a bad nonce)
When running the real transactions, every 4th transaction was bad as well in the same way

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[jbencin]

The transaction in question is not invalid, just partially signed (a 2-of-3 multisig with only 1 signature). So the question is why didn't it get signed a second time like the other txs? Possibly no matching pubkey => path mapping in your CSV file?

But yeah this error message is misleading, I will see if I can clean it up