[registrar] Improve an error message. (#1047)

docs/website/mtouch-errors.md

@@ -1134,6 +1134,10 @@ The selector for the property in question is not a valid Objective-C identifer.
 
 Please use a valid Objective-C identifier as selectors.
 
+<h3><a name="MT4165"/>MT4165: The registrar couldn't find the type 'System.Void' in any of the referenced assemblies.</h3>
+
+This error most likely indicates a bug in Xamarin.iOS. Please file a bug report at [http://bugzilla.xamarin.com](https://bugzilla.xamarin.com/enter_bug.cgi?product=iOS).
+
 <!--
  MT5xxx GCC and toolchain
   MT51xx compilation

tools/common/StaticRegistrar.cs

@@ -850,7 +850,7 @@ namespace XamCore.Registrar {
 					return type;
 			}
 
-			throw new Exception ("Couldn't find System.Void");
+			throw ErrorHelper.CreateError (4165, "The registrar couldn't find the type 'System.Void' in any of the referenced assemblies.");
 		}
 
 		protected override bool IsVirtual (MethodDefinition method)

tools/mtouch/error.cs

@@ -285,6 +285,7 @@ namespace Xamarin.Bundler {
 	//					MT4162	The type '{0}' (used as {1} {2}) is not available in {3} {4} (it was introduced in {3} {5}){6} Please build with a newer iOS SDK (usually done by using the most recent version of Xcode.
 	//					MT4163	Internal error in the registrar ({0}). Please file a bug report at http://bugzilla.xamarin.com
 	//					MT4164	Cannot export the property '{0}' because its selector '{1}' is an Objective-C keyword. Please use a different name.
+	//					MT4165	The registrar couldn't find the type 'System.Void' in any of the referenced assemblies.
 	// MT5xxx	GCC and toolchain
 	//			MT51xx	compilation
 	//					MT5101	Missing '{0}' compiler. Please install Xcode 'Command-Line Tools' component