Add invalidAPIKey case to GenerateContentError

[andrewheard]

Added an invalidAPIKey case to GenerateContentError to allow developers to catch invalid API key errors at runtime (e.g., when a key is revoked).

Example usage:

do {
  let content = try await model.generateContent("Why is the sky blue?")
} catch GenerateContentError.invalidAPIKey {
  print("The provided API key is invalid.")
}