fix: resetAccount should reject the promise with a `notFound` error if the account is not found.

Updating the fxa-auth-server to use the fxa-auth-db-mem instead of the Heap DB introduced a subtle change in resetAccount. The Heap DB rejected the promise with an `unknownAccount` error if the account's uid was not found, the fxa-auth-db-mem equivalent does not.

This change checks whether an account exists, if it does not, reject the promise with a `notFound` error.

fixes https://github.com/mozilla/fxa-auth-server/issues/773
Causes all tests referenced in https://github.com/mozilla/fxa-content-server/pull/1455 to pass.