Repositories use their associated HostingServiceAccount to fetch hosting

services. The hosting service account will return None if a hosting service

fails to load. This isn't the greatest thing to do since this hides the fact

that the service didn't load, and instead makes the repository behave as if

they do not have a hosting service configured.
This change instead raises an error when a HostingServiceAccount fails to

load a service. This way, a more appropriate and helpful eror message is shown

when, for example, trying to post changes or view diffs linked to repositories

that use a hosting service that can't be loaded.
This also adds some help texts to the HostingServiceAccount's fields. This

was primarily done to clarify that the service_name field should be set to

the hosting service ID rather than the hosting service name. This assumption

is made everywhere in the code except for in one test file which sets the

field to a hosting service name, which we correct in this change.

Ran all unit tests.
Tested viewing review requests and diffs that are from a repository that uses

  a hosting service that can't be loaded, saw the HostingServiceError page

  instead.
Tested rbt post REVISION with a git revision, saw a 

500: Internal Server Error response and saw the HostingServiceError get

  raised in the server logs.