Return proper exit code for errors

Author: jnm2

src/TfvcMigrator/InterceptableCommandHandler.cs

@@ -86,6 +86,11 @@ public static Delegate Intercept(Delegate @delegate)
             {
                 generator.Emit(OpCodes.Call, typeof(Task).GetProperty(nameof(Task.CompletedTask), BindingFlags.Public | BindingFlags.Static | BindingFlags.DeclaredOnly)!.GetMethod!);
             }
+            else if (@delegate.Method.ReturnType == typeof(Task<int>))
+            {
+                generator.Emit(OpCodes.Ldc_I4_0);
+                generator.Emit(OpCodes.Call, typeof(Task).GetMethod(nameof(Task.FromResult), BindingFlags.Public | BindingFlags.Static | BindingFlags.DeclaredOnly)!.MakeGenericMethod(typeof(int)));
+            }
             else
             {
                 throw new NotImplementedException();

src/TfvcMigrator/Program.cs

@@ -12,7 +12,7 @@ namespace TfvcMigrator;
 
 public static class Program
 {
-    public static Task Main(string[] args)
+    public static Task<int> Main(string[] args)
     {
         var command = new RootCommand("Migrates TFVC source history to idiomatic Git history while preserving branch topology.")
         {
@@ -58,7 +58,7 @@ private static RootPathChange ParseRootPathChange(string token)
         return new RootPathChange(changeset, token[(colonIndex + 1)..]);
     }
 
-    public static async Task MigrateAsync(
+    public static async Task<int> MigrateAsync(
         Uri projectCollectionUrl,
         string rootPath,
         string authors,
@@ -78,7 +78,7 @@ public static async Task MigrateAsync(
         if (Directory.GetFileSystemEntries(outputDirectory).Any())
         {
             Console.WriteLine($"Cannot create Git repository at {outputDirectory} because the directory is not empty.");
-            return;
+            return 1;
         }
 
         var authorsLookup = LoadAuthors(authors);
@@ -131,7 +131,7 @@ pat is not null
             Console.WriteLine("An entry must be added to the authors file for each of the following TFVC users:");
             foreach (var user in unmappedAuthors)
                 Console.WriteLine(user);
-            return;
+            return 1;
         }
 
         Console.WriteLine("Downloading changesets and converting to commits...");
@@ -336,6 +336,7 @@ pat is not null
         }
 
         Console.WriteLine($"\rAll {changesets.Count} changesets migrated successfully.");
+        return 0;
     }
 
     private static ImmutableDictionary<BranchIdentity, ImmutableArray<(string GitRepositoryPath, TfvcItem DownloadSource)>> MapItemsToDownloadSources(