"utils/get transport" test failure with Node 10

[stennie]

Failing test on Node 10 via https://travis-ci.org/keystonejs/keystone-email/jobs/386955263

1. utils
   get transport
   should throw if no transport was found:
   TypeError [ERR_AMBIGUOUS_ARGUMENT]: The "error/message" argument is ambiguous. The error message "Could not load transport (watermelon)" is identical to the message.
   at expectsError (assert.js:487:15)
   at Function.throws (assert.js:548:3)
   at Context. (tests/index.js:47:11)