add validation for configuration

lib/diaspora_federation.rb

@@ -34,7 +34,39 @@ module DiasporaFederation
     #   end
     def configure
       yield self
+      validate_config
       logger.info "successfully configured the federation engine"
     end
+
+    private
+
+    def validate_config
+      raise ConfigurationError, "missing server_uri" unless @server_uri.respond_to? :host
+      validate_class(@person_class, "person_class", %i(
+        find_local_by_diaspora_handle
+        guid
+        url
+        diaspora_handle
+        serialized_public_key
+        salmon_url
+        atom_url
+        profile_url
+        hcard_url
+      ))
+    end
+
+    def validate_class(klass, name, methods)
+      raise ConfigurationError, "missing #{name}" unless klass
+      const = const_get(klass)
+      methods.each {|method|
+        valid = const.respond_to?(method) || const.column_names.include?(method.to_s) || const.method_defined?(method)
+        raise ConfigurationError, "the configured class #{const} for #{name} does not respond to #{method}" unless valid
+      }
+    end
+  end
+
+  ##
+  # raised, if the engine is not configured correctly
+  class ConfigurationError < RuntimeError
   end
 end