[msbuild] Fix parsing of json parser errors to handle trailing periods in the error message.

Fixes this test:

    1) Test Failure : Xamarin.iOS.Tasks.Bug60536.TestACToolTaskCatchesJsonException
         ColumnNumber
      Expected: 2
      But was:  0

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

@@ -390,12 +390,15 @@ namespace Xamarin.MacDev.Tasks
 						int line = 0, column = 0;
 						int index, endIndex;
 
-						if ((index = ex.Message.IndexOf ("At line ", StringComparison.Ordinal)) != -1) {
+						var message = ex.Message;
+						if (message.EndsWith (".", StringComparison.Ordinal))
+							message = message.Substring (0, message.Length - 1);
+						if ((index = message.IndexOf ("At line ", StringComparison.Ordinal)) != -1) {
 							index += "At line ".Length;
 
-							if ((endIndex = ex.Message.IndexOf (", column ", index, StringComparison.Ordinal)) != -1) {
-								var columnBuf = ex.Message.Substring (endIndex + ", column ".Length);
-								var lineBuf = ex.Message.Substring (index, endIndex - index);
+							if ((endIndex = message.IndexOf (", column ", index, StringComparison.Ordinal)) != -1) {
+								var columnBuf = message.Substring (endIndex + ", column ".Length);
+								var lineBuf = message.Substring (index, endIndex - index);
 
 								int.TryParse (columnBuf, out column);
 								int.TryParse (lineBuf, out line);

msbuild/tests/Xamarin.iOS.Tasks.Tests/ProjectsTests/Bug60536.cs

@@ -71,7 +71,7 @@ namespace Xamarin.iOS.Tasks
 			Assert.AreEqual (2, Engine.Logger.ErrorEvents[0].ColumnNumber, "ColumnNumber");
 			Assert.AreEqual (197, Engine.Logger.ErrorEvents[0].EndLineNumber, "EndLineNumber");
 			Assert.AreEqual (2, Engine.Logger.ErrorEvents[0].EndColumnNumber, "EndColumnNumber");
-			Assert.AreEqual ("Unexpected character ']'. At line 197, column 2", Engine.Logger.ErrorEvents[0].Message, "Message");
+			Assert.AreEqual ("Unexpected character ']'. At line 197, column 2.", Engine.Logger.ErrorEvents[0].Message, "Message");
 		}
 	}
 }