[msbuild] Improve altool task by logging execution errors (#6815)

* [msbuild] Improve altool task by logging execution errors
The altool task was just logging the XML output produced by the tool execution but was not logging any build error neither failing in that case.

* [msbuild] Log the altool output when failing to parse it

msbuild/Xamarin.MacDev.Tasks.Core/Tasks/AlToolTaskBase.cs

@@ -3,12 +3,14 @@
 
 using Microsoft.Build.Utilities;
 using Microsoft.Build.Framework;
+using System.Text;
 
 namespace Xamarin.MacDev.Tasks
 {
 	public abstract class ALToolTaskBase : ToolTask
 	{
 		string sdkDevPath;
+		StringBuilder toolOutput;
 
 		public string SessionId { get; set; }
 
@@ -40,6 +42,17 @@ string DevicePlatformBinDir {
 			get { return Path.Combine (SdkDevPath, "usr", "bin"); }
 		}
 
+		public override bool Execute ()
+		{
+			toolOutput = new StringBuilder ();
+
+			base.Execute ();
+
+			LogErrorsFromOutput (toolOutput.ToString ());
+
+			return !HasLoggedErrors;
+		}
+
 		protected override string GenerateFullPathToTool ()
 		{
 			if (!string.IsNullOrEmpty (ToolPath))
@@ -70,6 +83,7 @@ protected override string GenerateCommandLineCommands ()
 
 		protected override void LogEventsFromTextOutput (string singleLine, MessageImportance messageImportance)
 		{
+			toolOutput.Append (singleLine);
 			Log.LogMessage (messageImportance, "{0}", singleLine);
 		}
 
@@ -82,5 +96,28 @@ string GetFileTypeValue ()
 				default: throw new NotSupportedException ($"Provided file type '{FileType}' is not supported by altool");
 			}
 		}
+
+		void LogErrorsFromOutput (string output)
+		{
+			try {
+				if (string.IsNullOrEmpty (output))
+					return;
+
+				var plist = PObject.FromString (output) as PDictionary;
+				var errors = PObject.Create (PObjectType.Array) as PArray;
+				var message = PObject.Create (PObjectType.String) as PString;
+
+				if ((plist?.TryGetValue ("product-errors", out errors) == true)) {
+					foreach (var error in errors) {
+						var dict = error as PDictionary;
+						if (dict?.TryGetValue ("message", out message) == true) {
+							Log.LogError (ToolName, null, null, null, 0, 0, 0, 0, "{0}", message.Value);
+						}
+					}
+				}
+			} catch (Exception ex) {
+				Log.LogWarning ($"Failed to parse altool output: {ex.Message}. \nOutput: {output}");
+			}
+		}
 	}
 }