fix: clarify EPLUGINCONF error message

The message now specify if the step is required and if it allows to configure multiple plugins.

lib/plugins/index.js

@@ -2,6 +2,7 @@ const {identity, isPlainObject, omit, castArray, isUndefined} = require('lodash'
 const AggregateError = require('aggregate-error');
 const getError = require('../get-error');
 const PLUGINS_DEFINITIONS = require('../definitions/plugins');
+const {validateConfig} = require('./utils');
 const pipeline = require('./pipeline');
 const normalize = require('./normalize');
 
@@ -11,7 +12,7 @@ module.exports = (context, pluginsPath) => {
   const plugins = Object.entries(PLUGINS_DEFINITIONS).reduce(
     (
       plugins,
-      [type, {configValidator, default: def, pipelineConfig, postprocess = identity, preprocess = identity}]
+      [type, {multiple, required, default: def, pipelineConfig, postprocess = identity, preprocess = identity}]
     ) => {
       let pluginOpts;
 
@@ -23,8 +24,8 @@ module.exports = (context, pluginsPath) => {
         if (isPlainObject(options[type]) && !options[type].path && defaultPaths.length === 1) {
           [options[type].path] = defaultPaths;
         }
-        if (configValidator && !configValidator(options[type])) {
-          errors.push(getError('EPLUGINCONF', {type, pluginConf: options[type]}));
+        if (!validateConfig({multiple, required}, options[type])) {
+          errors.push(getError('EPLUGINCONF', {type, multiple, required, pluginConf: options[type]}));
           return plugins;
         }
         pluginOpts = options[type];

lib/plugins/utils.js

@@ -0,0 +1,18 @@
+const {isString, isFunction, castArray} = require('lodash');
+
+const validateSingleConfig = conf => {
+  conf = castArray(conf);
+  return conf.length === 1 && (isString(conf[0]) || isString(conf[0].path) || isFunction(conf[0]));
+};
+
+const validateMultipleConfig = conf => castArray(conf).every(conf => validateSingleConfig(conf));
+
+const validateConfig = ({multiple, required}, conf) => {
+  conf = castArray(conf).filter(Boolean);
+  if (required) {
+    return Boolean(conf) && conf.length >= 1 && (multiple ? validateMultipleConfig : validateSingleConfig)(conf);
+  }
+  return conf.length === 0 || (multiple ? validateMultipleConfig : validateSingleConfig)(conf);
+};
+
+module.exports = {validateConfig};