Fix set command

src/main/java/com/maddyhome/idea/vim/vimscript/model/commands/SetCommand.kt

@@ -143,6 +143,8 @@ fun parseOptionLine(editor: Editor, args: String, scope: OptionService.Scope, fa
         if (eq == -1) {
           if (optionService.isToggleOption(token)) {
             optionService.setOption(scope, token, token)
+          } else if (!optionService.getOptions().contains(token)) {
+            error = Msg.unkopt
           } else {
             toShow.add(Pair(token, token))
           }

src/test/java/org/jetbrains/plugins/ideavim/ex/implementation/commands/SetCommandTest.kt

@@ -0,0 +1,31 @@
+/*
+ * IdeaVim - Vim emulator for IDEs based on the IntelliJ platform
+ * Copyright (C) 2003-2021 The IdeaVim authors
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+
+package org.jetbrains.plugins.ideavim.ex.implementation.commands
+
+import org.jetbrains.plugins.ideavim.VimTestCase
+
+class SetCommandTest : VimTestCase() {
+
+  fun `test unknown option`() {
+    configureByText("\n")
+    typeText(commandToKeys("set unknownOption"))
+    assertPluginError(true)
+    assertPluginErrorMessageContains("Unknown option: unknownOption")
+  }
+}