Follow up error while processing response of an unauthorized request

[eric-christian]

Hello

I have noticed that when a request is made without proper authentication, the processing of the 401 response causes an exception.
There is a message attribute expected within the response which will be read into api_message. But the json response does not contain a message attribute. So api_message will be set to nil.
https://github.com/mailgun/mailgun-ruby/blob/master/lib/mailgun/exceptions/exceptions.rb#L53

Expected behavior

To have a Mailgun::CommunicationError raised.

Actual behavior

A TypeError is raised.

How to reproduce

To reproduce, do the following:
irb> Mailgun::Client.new('key-invalid').delete('my-random-domain/bounces/[email protected]')

/Users/eric/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/mailgun-ruby-1.2.10/lib/mailgun/exceptions/exceptions.rb:63:in `+': no implicit conversion of nil into String (TypeError)

      message = message + ': ' + api_message
                                 ^^^^^^^^^^^

Environment

ruby: 3.2.2
mailgun: 1.2.10
rails: 7.0.7.2