Create get_rest_client method

This change creates get_rest_client, to avoid us having to repeatedly
get the same RestClient.  It also allows us to easier move to another
rest client with less changes.

We also remove all the error messages since saying (for example) that
'No categories found' when you get a non-200 response is silly.  In
actual fact, we should never see this as non-200s throw an exception.