[dotnet-linker] Improve the error message from the linker when we run…

… into a problem. (xamarin#14253)

* Better text.
* Better number ("7000" is more like "this is kind of normal" - while "6999"
  was more like "something quite expected happened").
* Use resources to make it localizable.

tools/dotnet-linker/LinkerConfiguration.cs

@@ -457,10 +457,9 @@ public static void Report (LinkContext context, IList<Exception> exceptions)
 			var list = ErrorHelper.CollectExceptions (exceptions);
 			var allWarnings = list.All (v => v is ProductException pe && !pe.Error);
 			if (!allWarnings) {
-				// Revisit the error code after https://github.com/mono/linker/issues/1596 has been fixed.
 				var instance = GetInstance (context, false);
 				var platform = (instance?.Platform)?.ToString () ?? "unknown";
-				var msg = MessageContainer.CreateCustomErrorMessage ("Failed to execute the custom steps.", 6999, platform);
+				var msg = MessageContainer.CreateCustomErrorMessage (Errors.MX7000 /* An error occured while executing the custom linker steps. Please review the build log for more information. */, 7000, platform);
 				context.LogMessage (msg);
 			}
 			// ErrorHelper.Show will print our errors and warnings to stderr.

tools/mtouch/Errors.resx

@@ -2114,6 +2114,13 @@
 		</value>
 	</data>
 
+	<!-- The linker says custom linker steps can't use error codes between 1000 and 6000, so we use >= 7000 -->
+	<data name="MX7000" xml:space="preserve">
+		<value>An error occured while executing the custom linker steps. Please review the build log for more information.</value>
+	</data>
+
+	<!-- 8XXX are for errors that occur at runtime -->
+
 	<data name="MT8018" xml:space="preserve">
 		<value>Internal consistency error. Please file a bug report at https://github.com/xamarin/xamarin-macios/issues/new.
 		</value>