The problem is _getErrorWithCode(), compiled into the shell from utils.js. That function expects the provided error object to be a command failure with ok: 0, or an object with a "writeError" field, not a "writeErrors" array.
I think _getErrorWithCode() should check for writeErrors, too. I'm not sure "writeError" is ever actually present.
This is a problem for tests that check things like "ex.code === 123". If the command failed with a writeErrors array, the exception's code is an object, not the expected number, and the check may be false incorrectly.