include HelpText in CommandParsingError, write it to stdout

src/dotnet/CommandLine/CommandParsingException.cs

@@ -10,8 +10,11 @@ namespace Microsoft.DotNet.Cli.CommandLine
     {
         private readonly bool _isRequireSubCommandMissing;
 
-        public CommandParsingException(string message) : base(message)
+        public CommandParsingException(
+            string message, 
+            string helpText = null) : base(message)
         {
+            HelpText = helpText ?? "";
             Data.Add("CLI_User_Displayed_Exception", true);
         }
 
@@ -27,6 +30,8 @@ namespace Microsoft.DotNet.Cli.CommandLine
 
         public CommandLineApplication Command { get; }
 
+        public string HelpText { get; } = "";
+
         public override string Message
         {
             get

src/dotnet/ParseResultExtensions.cs

@@ -27,8 +27,9 @@ namespace Microsoft.DotNet.Cli
             if (parseResult.Errors.Any())
             {
                 throw new CommandParsingException(
-                    string.Join(Environment.NewLine,
+                    message: string.Join(Environment.NewLine,
-                                parseResult.Errors.Select(e => e.Message)));
+                                parseResult.Errors.Select(e => e.Message)),
+                    helpText: parseResult?.Command()?.HelpView());
             }
         }
     }

src/dotnet/Program.cs

@@ -5,6 +5,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text;
+using Microsoft.DotNet.Cli.CommandLine;
 using Microsoft.DotNet.Cli.Utils;
 using Microsoft.DotNet.Configurer;
 using Microsoft.DotNet.PlatformAbstractions;
@@ -84,9 +85,15 @@ namespace Microsoft.DotNet.Cli
             }
             catch (Exception e) when (e.ShouldBeDisplayedAsError())
             {
-                Reporter.Error.WriteLine(CommandContext.IsVerbose() ? 
+                Reporter.Error.WriteLine(CommandContext.IsVerbose() 
-                        e.ToString().Red().Bold() : 
+                    ? e.ToString().Red().Bold() 
-                        e.Message.Red().Bold());
+                    : e.Message.Red().Bold());
+
+                var commandParsingException = e as CommandParsingException;
+                if (commandParsingException != null)
+                {
+                    Reporter.Output.WriteLine(commandParsingException.HelpText);
+                }
 
                 return 1;
             }

src/dotnet/commands/dotnet-sln/SlnCommandParser.cs

@@ -20,7 +20,7 @@ namespace Microsoft.DotNet.Cli
                 CommonOptions.HelpOption(),
                 Create.Command("add",
                                ".NET Add project(s) to a solution file Command",
-                               Accept.OneOrMoreArguments()
+                               Accept.OneOrMoreArguments(o => CommonLocalizableStrings.SpecifyAtLeastOneProjectToAdd)
                                      .With(name: "args",
                                            description: LocalizableStrings.AddSubcommandHelpText),
                                CommonOptions.HelpOption()),