Fall back to other methods of schema loading when one fails

[shadaj]

Helps in situations where one method fails like when an endpoint exists but introspection is disabled.

[danilobuerger]

@shadaj this now leads to unclear error messages when fetching from a server without introspection.
Error message is now:

$ apollo schema:download --endpoint ... schema.json
 ✔ Loading Apollo config
 ✔ Fetching current schema
 ✖ Saving schema to schema.json
   → Expected undefined to be a GraphQL schema.
Error: Expected undefined to be a GraphQL schema.

previously it was:

$ apollo schema:download --endpoint ... schema.json
 ✔ Loading Apollo config
 ✖ Fetching current schema
   → Response not successful: Received status code 400
   Saving schema to schema.json
Error: Response not successful: Received status code 400

[abernix]

@shadaj I've proposed reverting this in #613. I'd appreciate your thoughts on that, but it seems like this approach won't work.