remove addressed FIXMEs in update_auth

[minrk]

These are both addressed by the use of traitlets.config:

• an informative error is raised if the auth class cannot be imported
• only configurable traits on Configurable classes can be set this way, not any attribute on any object

A check for importability of the auth config class is appropriate, but not here, during the loading of config, where there is already a check when JupyterHub attempts to load the auth class. It would be appropriate during tljh-config which sets the config, however.

also fix a couple of typos in the docstring that I introduced during review of #721

Closes #740 (edit by Erik)

[yuvipanda]

Thanks a lot for bearing with me, @minrk and @consideRatio!