Set the failed http response in a custom attribute instead of cause

`#cause` in a custom error should return the expected class. This
commit updates the custom value that is useful to store, the response,
to use a specific name rather than overriding `#cause`’s behavior.

Ruby documentation explains that the default expectation of #cause is
that it will return either `an_exception` or `nil`. By setting an
`HTTP::Response` to return from `#cause`, tools utilizing it may expect
to send messages that may fail.

https://ruby-doc.org/3.2.6/Exception.html#method-i-cause

Specifically, I experienced a `NoMethodError: undefined method
'backtrace'` when an error occurred within `pry`.

lib/peddler/error.rb

@@ -7,7 +7,7 @@ class NotFound < Error; end
     class QuotaExceeded < Error; end
     class Unauthorized < Error; end
 
-    attr_reader :cause
+    attr_reader :response
 
     # @!visibility private
     class << self
@@ -23,8 +23,8 @@ def build(response)
       end
     end
 
-    def initialize(msg = nil, cause = nil)
-      @cause = cause
+    def initialize(msg = nil, response = nil)
+      @response = response
       super(msg)
     end
   end