throw exceptions for command not found

src/dotnet/DotNetTopLevelCommandBase.cs

@@ -6,6 +6,7 @@ using System.Collections.Generic;
 using System.Linq;
 using Microsoft.DotNet.Cli.CommandLine;
 using Microsoft.DotNet.Cli.Utils;
+using Microsoft.DotNet.Tools;
 
 namespace Microsoft.DotNet.Cli
 {
@@ -29,14 +30,18 @@ namespace Microsoft.DotNet.Cli
 
             var subcommandName = result.Command().Name;
 
-            var create = SubCommands[subcommandName];
-
-            var command = create(result["dotnet"][CommandName]);
-
             try
             {
+                var create = SubCommands[subcommandName];
+
+                var command = create(result["dotnet"][CommandName]);
+
                 return command.Execute();
             }
+            catch (KeyNotFoundException e)
+            {
+                throw new GracefulException(CommonLocalizableStrings.RequiredCommandNotPassed);
+            }
             catch (GracefulException e)
             {
                 Reporter.Error.WriteLine(e.Message.Red());

src/dotnet/HelpException.cs

@@ -9,15 +9,9 @@ namespace Microsoft.DotNet.Cli
     [Obsolete("This is intended to facilitate refactoring during parser replacement and should not be used after that work is done.")]
     public class HelpException : Exception
     {
-        public HelpException(
+        public HelpException(string message) : base(message)
-            string message,
-            bool isError = false) : base(message)
         {
-            IsError = isError;
-
             Data.Add(ExceptionExtensions.CLI_User_Displayed_Exception, true);
         }
-
-        public bool IsError { get; }
     }
 }

src/dotnet/Program.cs

@@ -80,7 +80,7 @@ namespace Microsoft.DotNet.Cli
             catch (HelpException e)
             {
                 Reporter.Output.Write(e.Message);
-                return e.IsError ? 1 : 0;
+                return 0;
             }
             catch (Exception e) when (e.ShouldBeDisplayedAsError())
             {

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

@@ -12,7 +12,6 @@ namespace Microsoft.DotNet.Cli
                 "sln",
                 ".NET modify solution file command",
                 Accept.ExactlyOneArgument()
-                      .ExistingSlnFileOrDirectoryOnly()
                       .DefaultToCurrentDirectory()
                       .With(name: "SLN_FILE"),
                 CommonOptions.HelpOption(),