update docs/sdk/error handling to reference new demo endpoint

[OwenPriceSkelly]

Before the dlhub endpoint can be switched off, we need to update docs and the sdk in light of the new requirement that users join the correct globus group to use the new demo endpoint.

Assumptions:

1. the new demo endpoint is functional (#418)
2. demo endpoint is only available to members of the Garden Users globus group here (#425)
3. no manual approval is needed for users to join the group, just the join link

Acceptance Criteria:

• docs/tutorial have a hard-to-miss link to join the group (like MDF does)
• entrypoints run remotely on demo endpoint by default
• if the user has not yet joined the group when they try to run an entrypoint, the error message also gives them the link to join