fix(account): filter account.toJSON
refactor(client): Simplify the error handling code.

* Rename `code` to `errno` to be consistent with the backend.
* Use a single hash called ERRORS for each namespace. The has contains both `errno` and `message`
* Extract `errors.js` from logic that was in `auth-errors.js`
* Extract `profile-errors.js` from logic in `profile-client.js`

fixes #1508
fix(docs): update AUTHORS list

Closes #1981
Merge pull request #1976 from mozilla/issue-1508-refactor-errors

refactor(client): Simplify the error handling code.
Merge pull request #1990 from mozilla/issue-1975-poluted-json

fix(account): filter account.toJSON
fix(client): We broke password managers with `autocomplete=off` on password fields!

* Always add `autocomplete=off` when the relier is Sync.
* Add `autocomplete=off` to text fields acting as passwords, otherwise passwords are displayed in a form autocomplete dropdown. Yuck.

fixes #1788