fix: Ensure to report validation errors with ServerlessError

lib/Serverless.js

@@ -29,6 +29,7 @@ class Serverless {
     this.configurationPath = ensureString(configObject.configurationPath, {
       isOptional: true,
       name: 'config.configurationPath',
+      Error: ServerlessError,
     });
     if (this.configurationPath) {
       this.configurationPath = path.resolve(this.configurationPath);