[xcode9.4][msbuild] Catch ArgumentException if IBToolTask fails to load plist o… (#3940)

Fixes the stack trace spew in https://devdiv.visualstudio.com/DevDiv/_workitems/edit/595179

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

@@ -191,7 +191,7 @@ namespace Xamarin.MacDev.Tasks
 						var plist = PDictionary.FromFile (manifest.ItemSpec);
 
 						LogWarningsAndErrors (plist, items[0]);
-					} catch (FormatException) {
+					} catch {
 					}
 
 					File.Delete (manifest.ItemSpec);