[ruby] Fix api_error.mustache to initialize message-only errors properly

modules/openapi-generator/src/main/resources/ruby-client/api_error.mustache

@@ -24,6 +24,7 @@ module {{moduleName}}
         end
       else
         super arg
+        @message = arg
       end
     end
 

samples/client/petstore/ruby-autoload/lib/petstore/api_error.rb

@@ -32,6 +32,7 @@ def initialize(arg = nil)
         end
       else
         super arg
+        @message = arg
       end
     end
 

samples/client/petstore/ruby-faraday/lib/petstore/api_error.rb

@@ -32,6 +32,7 @@ def initialize(arg = nil)
         end
       else
         super arg
+        @message = arg
       end
     end
 

samples/client/petstore/ruby/lib/petstore/api_error.rb

@@ -32,6 +32,7 @@ def initialize(arg = nil)
         end
       else
         super arg
+        @message = arg
       end
     end
 

samples/openapi3/client/extensions/x-auth-id-alias/ruby-client/lib/x_auth_id_alias/api_error.rb

@@ -32,6 +32,7 @@ def initialize(arg = nil)
         end
       else
         super arg
+        @message = arg
       end
     end
 

samples/openapi3/client/features/dynamic-servers/ruby/lib/dynamic_servers/api_error.rb

@@ -32,6 +32,7 @@ def initialize(arg = nil)
         end
       else
         super arg
+        @message = arg
       end
     end
 

samples/openapi3/client/features/generate-alias-as-model/ruby-client/lib/petstore/api_error.rb

@@ -32,6 +32,7 @@ def initialize(arg = nil)
         end
       else
         super arg
+        @message = arg
       end
     end