fix(generator): Fix crash when a response doesn't reference a type

[dazuma]

Includes two fixes:

• We include more stringent checks in the generator to handle certain types of malformed discovery docs more gracefully. Specifically, these are methods that don't specify a proper type or specify a nonexistent type for their request or response. As of this writing, integrations_v1 and identitytoolkit_v1 exhibit these kinds of errors.
• We recover better during a weekly regeneration job if an API does fail generation catastrophically. Instead of causing the entire job to abort prematurely, we just note the error and skip the offending API.