Make error reporting consistent whenever throwing an exception in a Command

Right now, if you throw a UserAssertion in the body of a command, the result object you get back from the command looks like:
{ok: 0,
code: <assertion code>,
errmsg: <assertion message>
}

If you throw an exception from some other part of the command code other than the run method (for example in the access control check or the parseNs method), you get back something like:
{ok: 0,
assertionCode: <assertion code>,
assertion: <assertion message>,
errmsg: "db assertion failure"
}

We should make these consistent