output IOSBuildTools when it doesnt exist (#355)

Author: SimonCropp

src/Sentry.Unity.Editor.iOS/Sentry.Unity.Editor.iOS.csproj

@@ -21,7 +21,7 @@
       <IOSBuildTools>$(UnityRoot)/Data/PlaybackEngines/iOSSupport/UnityEditor.iOS.Extensions.Xcode.dll</IOSBuildTools>
     </PropertyGroup>
 
-    <Error Condition="!Exists('$(IOSBuildTools)')" Text="iOS build tools not found. Make sure to install the Unity iOS Module."></Error>
+    <Error Condition="!Exists('$(IOSBuildTools)')" Text="iOS build tools not found. Make sure to install the Unity iOS Module. Expected directory: $(IOSBuildTools)"></Error>
 
     <ItemGroup>
       <Reference Include="UnityEditor">