Access error details through StreamAPIException class attr_readers (#56)

* add accessors and to_s to StreamAPIException class

* remove resonse attr reader, and add json_response?

lib/stream-chat/errors.rb

@@ -4,6 +4,9 @@
 
 module StreamChat
   class StreamAPIException < StandardError
+    attr_reader :error_code
+    attr_reader :error_message
+
     def initialize(response)
       super()
       @response = response
@@ -24,6 +27,14 @@ def message
         "StreamChat error HTTP code: #{@response.status}"
       end
     end
+
+    def json_response?
+      @json_response
+    end
+
+    def to_s
+      message
+    end
   end
 
   class StreamChannelException < StandardError; end