[msbuild] Improve DetectSdkLocationsTaskBase error log (#2012)

First: only run `EnsureSdkPath` if `EnsureAppleSdkRoot` passed.
    Both are based on similar install checks, `SdkIsInstalled` (used for `SdkIsInstalled`) is empty
    if `IsInstalled` (used by `EnsureAppleSdkRoot`) is false.
    This avoid having 2 error messages when only 1 at a time is needed.

Second: improved `EnsureAppleSdkRoot` error message mentioning the wrong Xcode developer path (which triggered the error)
    as well as the exact way to fix it.

external/Xamarin.MacDev

@@ -1 +1 @@
-Subproject commit e7b0f35eb69b579f202d842aeec0d6a8afcdc357
+Subproject commit 2688b706e040ff17dd3a6417f847dadee96504eb

msbuild/Xamarin.Mac.Tasks.Core/Tasks/DetectSdkLocationsTaskBase.cs

@@ -61,9 +61,9 @@ namespace Xamarin.Mac.Tasks
 			Log.LogTaskName ("DetectSdkLocations");
 			Log.LogTaskProperty ("XamarinSdkRoot", XamarinSdkRoot);
 
-			EnsureAppleSdkRoot ();
-			EnsureXamarinSdkRoot ();
+			if (EnsureAppleSdkRoot ())
 				EnsureSdkPath ();
+			EnsureXamarinSdkRoot ();
 
 			IsXcode8 = AppleSdkSettings.XcodeVersion.Major >= 8;
 
@@ -93,18 +93,25 @@ namespace Xamarin.Mac.Tasks
 				Log.LogError ("Could not locate SDK bin directory");
 		}
 
-		void EnsureAppleSdkRoot ()
+		bool EnsureAppleSdkRoot ()
 		{
 			if (!MacOSXSdks.Native.IsInstalled) {
-				Log.LogError ("Could not find a valid Xcode app bundle");
-			} else {
+				var ideSdkPath = "(Project > SDK Locations > Apple > Apple SDK)";
+#if WINDOWS
+				var ideSdkPath = "(Tools > Options > Xamarin > iOS Settings > Apple SDK)";
+#endif
+				Log.LogError ("Could not find a valid Xcode app bundle at '{0}'. Please update your Apple SDK location in Visual Studio's preferences {1}.", AppleSdkSettings.InvalidDeveloperRoot, ideSdkPath);
+				return false;
+			}
 			Log.LogMessage(MessageImportance.Low, "DeveloperRoot: {0}", MacOSXSdks.Native.DeveloperRoot);
 			Log.LogMessage(MessageImportance.Low, "GetPlatformPath: {0}", MacOSXSdks.Native.GetPlatformPath());
 
 			SdkDevPath = MacOSXSdks.Native.DeveloperRoot;
-				if (string.IsNullOrEmpty (SdkDevPath))
+			if (string.IsNullOrEmpty(SdkDevPath)) {
 				Log.LogError("Could not find a valid Xcode developer path");
+				return false;
 			}
+			return true;
 		}
 
 		void EnsureXamarinSdkRoot ()

msbuild/Xamarin.iOS.Tasks.Core/Tasks/DetectSdkLocationsTaskBase.cs

@@ -99,8 +99,7 @@ namespace Xamarin.iOS.Tasks
 
 			IsXcode8 = AppleSdkSettings.XcodeVersion.Major >= 8;
 
-			EnsureAppleSdkRoot ();
-			EnsureXamarinSdkRoot ();
+			if (EnsureAppleSdkRoot ()) {
 				switch (Framework) {
 				case PlatformFramework.iOS:
 					EnsureiOSSdkPath ();
@@ -114,6 +113,8 @@ namespace Xamarin.iOS.Tasks
 				default:
 					throw new InvalidOperationException (string.Format ("Invalid framework: {0}", Framework));
 				}
+			}
+			EnsureXamarinSdkRoot ();
 
 			Log.LogTaskName ("DetectSdkLocations");
 			Log.LogTaskProperty ("TargetFrameworkIdentifier", TargetFrameworkIdentifier);
@@ -287,19 +288,26 @@ namespace Xamarin.iOS.Tasks
 			SdkPlatform = SdkIsSimulator ? "iPhoneSimulator" : "iPhoneOS";
 		}
 
-		void EnsureAppleSdkRoot ()
+		bool EnsureAppleSdkRoot ()
 		{
 			if (!CurrentSdk.IsInstalled) {
-				Log.LogError ("  Could not find a usable Xcode app bundle in {0}", CurrentSdk.DeveloperRoot);
-			} else {
+				var ideSdkPath = "(Project > SDK Locations > Apple > Apple SDK)";
+#if WINDOWS
+				var ideSdkPath = "(Tools > Options > Xamarin > iOS Settings > Apple SDK)";
+#endif
+				Log.LogError ("Could not find a valid Xcode app bundle at '{0}'. Please update your Apple SDK location in Visual Studio's preferences {1}.", AppleSdkSettings.InvalidDeveloperRoot, ideSdkPath);
+				return false;
+			}
 			Log.LogMessage (MessageImportance.Low, "DeveloperRoot: {0}", CurrentSdk.DeveloperRoot);
 			Log.LogMessage (MessageImportance.Low, "DevicePlatform: {0}", CurrentSdk.DevicePlatform);
 			Log.LogMessage (MessageImportance.Low, "GetPlatformPath: {0}", CurrentSdk.GetPlatformPath (false));
 
 			SdkDevPath = CurrentSdk.DeveloperRoot;
-				if (string.IsNullOrEmpty (SdkDevPath))
+			if (string.IsNullOrEmpty (SdkDevPath)) {
 				Log.LogError ("Could not find valid a usable Xcode developer path");
+				return false;
 			}
+			return true;
 		}
 
 		void EnsureXamarinSdkRoot ()