Check for file/directory before creating cache dependency

When checking the VPP for resources that are handled by other virtual providers, GetCacheDependency throws HttpException with "Directory '[virtual path]' does not exist. Failed to start monitoring file changes." This change verifies that the resource exists to create a dependency on.

This is an alternate patch for the same issue as pull request https://github.com/mcintyre321/EmbeddedResourceVirtualPathProvider/pull/2