[tests] Improve an assert message slightly. (#14429)

tests/msbuild/Xamarin.MacDev.Tests/TestHelpers/TestBase.cs

@@ -333,8 +333,8 @@ namespace Xamarin.Tests
 			var rv = Engine.RunTarget (ApplePlatform, executionMode ?? Mode, paths.ProjectCSProjPath, target, properties);
 			if (expectedErrorCount != Engine.ErrorEvents.Count) {
 				foreach (var e in Engine.ErrorEvents)
-					Console.WriteLine (e.Message);
-				Assert.AreEqual (expectedErrorCount, Engine.ErrorEvents.Count, "ExitCode/ExpectedErrorCount");
+					Console.WriteLine (e.ToString ());
+				Assert.AreEqual (expectedErrorCount, Engine.ErrorEvents.Count, $"Unexpected number of errors when executing target '{target}'");
 			}
 			if (expectedErrorCount > 0) {
 				Assert.AreEqual (1, rv.ExitCode, "ExitCode (failure)");