Better error response to malformed headers (#35278)

lib/exceptions.rb

@@ -13,6 +13,7 @@ module Mastodon
   class SyntaxError < Error; end
   class InvalidParameterError < Error; end
   class SignatureVerificationError < Error; end
+  class MalformedHeaderError < Error; end
 
   class UnexpectedResponseError < Error
     attr_reader :response