[vim] Don't throw errors to signal missing options

Addresses codemirror#4771

Author: vincentwoo

keymap/vim.js

@@ -411,11 +411,11 @@
       cfg = cfg || {};
       var scope = cfg.scope;
       if (!option) {
-        throw Error('Unknown option: ' + name);
+        return new Error('Unknown option: ' + name);
       }
       if (option.type == 'boolean') {
         if (value && value !== true) {
-          throw Error('Invalid argument: ' + name + '=' + value);
+          return new Error('Invalid argument: ' + name + '=' + value);
         } else if (value !== false) {
           // Boolean options are set to true if value is not defined.
           value = true;
@@ -443,7 +443,7 @@
       cfg = cfg || {};
       var scope = cfg.scope;
       if (!option) {
-        throw Error('Unknown option: ' + name);
+        return new Error('Unknown option: ' + name);
       }
       if (option.callback) {
         var local = cm && option.callback(undefined, cm);
@@ -4300,13 +4300,18 @@
         // If no value is provided, then we assume this is a get.
         if (!optionIsBoolean && value === undefined || forceGet) {
           var oldValue = getOption(optionName, cm, setCfg);
-          if (oldValue === true || oldValue === false) {
+          if (oldValue instanceof Error) {
+            showConfirm(cm, oldValue.message);
+          } else if (oldValue === true || oldValue === false) {
             showConfirm(cm, ' ' + (oldValue ? '' : 'no') + optionName);
           } else {
             showConfirm(cm, '  ' + optionName + '=' + oldValue);
           }
         } else {
-          setOption(optionName, value, cm, setCfg);
+          var setOptionReturn = setOption(optionName, value, cm, setCfg);
+          if (setOptionReturn instanceof Error) {
+            showConfirm(cm, setOptionReturn.message);
+          }
         }
       },
       setlocal: function (cm, params) {