throw CommandParsingException rather than crash on missing single value

test/dotnet.Tests/CommandLineApplicationTests.cs

@@ -0,0 +1,26 @@
+using System;
+using FluentAssertions;
+using Microsoft.DotNet.Cli.CommandLine;
+using Xunit;
+
+namespace Microsoft.DotNet.Tests
+{
+    public class CommandLineApplicationTests
+    {
+        [Fact]
+        public void WhenAnOptionRequiresASingleValueThatIsNotSuppliedItThrowsCommandParsingException()
+        {
+            var app = new CommandLineApplication();
+
+            app.Option("-v|--verbosity", "be verbose", CommandOptionType.SingleValue);
+
+            Action execute = () => app.Execute("-v");
+
+            execute.ShouldThrow<CommandParsingException>()
+                   .Which
+                   .Message
+                   .Should()
+                   .Be("Required value for option '-v' was not provided.");
+        }
+    }
+}