Improve log on GenerateResource task

When investigating on why my incremental build was not that incremental I had a hard time understanding why a specific resource file was always recompiled. It took me hours to download / compile and understand correctly what was goind wrong.

In my case I was hitting: https://github.com/Microsoft/msbuild/blob/a9f64ebd108702c3fc65339c66cb124217854524/src/Tasks/GenerateResource.cs#L1428-L1433

But my file is not missing, the path is only badly formatted: ...\Resources\BuildTagIcon.png (notice the threed dots). For NativeMethodsShared.GetFileAttributesEx it's not valid but it's ok for the resource generator.

We sure can fix this issue but I guess it would be more useful to add some logs. Once I knew which resource was the culprit it was quite easy to understand why.