Refactoring: Move SignatureVerificationError into Mastodon namespace (#34342)

lib/exceptions.rb

@@ -12,6 +12,7 @@ module Mastodon
   class RateLimitExceededError < Error; end
   class SyntaxError < Error; end
   class InvalidParameterError < Error; end
+  class SignatureVerificationError < Error; end
 
   class UnexpectedResponseError < Error
     attr_reader :response