[tools] Fix mono-cil-strip to return non-zero on failure [#28617]

mcs/tools/cil-strip/cilstrip.cs

@@ -8,7 +8,6 @@
 //
 
 using System;
-using System.IO;
 using System.Reflection;
 
 using Mono.Cecil;
@@ -17,7 +16,7 @@ namespace Mono.CilStripper {
 
 	class Program {
 
-		static void Main (string [] args)
+		static int Main (string [] args)
 		{
 			Header ();
 
@@ -35,11 +34,13 @@ namespace Mono.CilStripper {
 					Console.WriteLine ("Assembly {0} stripped out into {1}", file, output);
 				else
 					Console.WriteLine ("Assembly {0} stripped", file);
+				return 0;
 			} catch (TargetInvocationException tie) {
 				Console.WriteLine ("Error: {0}", tie.InnerException);
 			} catch (Exception e) {
 				Console.WriteLine ("Error: {0}", e);
 			}
+			return 1;
 		}
 
 		static void StripAssembly (AssemblyDefinition assembly, string output)