code: function validateKeyword(definition, throwError) {
    validateKeyword.errors = null;
    var v = this._validateKeyword = this._validateKeyword
                                    || this.compile(definitionSchema, true);
  
    if (v(definition)) return true;
    validateKeyword.errors = v.errors;
    if (throwError)
      throw new Error('custom keyword definition is invalid: '  + this.errorsText(v.errors));
    else
      return false;
  }
docs: Validate keyword definition
  @this  Ajv
  @param {Object} definition keyword definition object.
  @param {Boolean} throwError true to throw exception if definition is invalid
  @return {boolean} validation result