ISSUE-1734 Add missing configuration warning

This PR adds a more descriptive error message when users add a detector
without configuration. Before this change a NoMethodError exception was
raised with an unclear error message. Now we catch this exception and
provide a more helpful error message.

A better future option might be add
[dry-schema](https://dry-rb.org/gems/dry-schema/1.13/)

See: troessner#1734

lib/reek/configuration/configuration_converter.rb

@@ -80,6 +80,8 @@ def strings_to_regexes_for_detectors
                 to_regex item
               end
             end
+          rescue NoMethodError
+            warn_about_missing_configuration(detector) if detectors[detector].nil?
           end
         end
       end
@@ -101,10 +103,25 @@ def strings_to_regexes_for_directories
                   to_regex item
                 end
               end
+            rescue NoMethodError
+              warn_about_missing_configuration(detector) if configuration.nil?
             end
           end
         end
       end
+
+      def warn_about_missing_configuration(detector)
+        msg = <<~ERROR
+          ###############################
+
+          Please review the configuration file (e.g. .reek.yml in your project directory).
+          It looks like the configuration for #{detector} is missing.
+          You can find the configuration options documentation over here: https://github.com/troessner/reek#configuration-options.
+
+          ###############################
+        ERROR
+        warn msg
+      end
     end
   end
 end

lib/reek/configuration/default_directive.rb

@@ -21,8 +21,10 @@ module DefaultDirective
       # @return [self]
       def add(detectors_configuration)
         detectors_configuration.each do |name, configuration|
-          detector = key_to_smell_detector(name)
-          self[detector] = (self[detector] || {}).merge configuration
+          if configuration
+            detector = key_to_smell_detector(name)
+            self[detector] = (self[detector] || {}).merge configuration
+          end
         end
         self
       end