Minor Request: assertUserError

[machineghost]

Currently without graphql-errors one can do:

const assert = require('assert');
assert(false, 'something was missing');

But of course, if you do that inside a graphql-errors call that error will get masked. If you want to assert in a way that shows the user what went wrong you need to make your own assert:

const { UserError } = require('graphql-errors');
const assertUserError = (condition, message) => { 
    if(!condition) throw new UserError(message);
};

Obviously it's not a big deal to write the above function yourself, but since it's basically just a one-liner I thought it might be nice if such a method was provided as a convenience by the library:

const {  assertUserError } = require('graphql-errors');
assertUserError(false, 'something was missing');

If you think such a function is within the library's scope I'd be happy to provide a PR.