Directive usage errors need to appear in runtime too

[ajaybhargavb]

There are cases like aspnet/Razor#2539 (comment) when using directives as variables in cshtml results in a weird csharp error at runtime. Instead we should provide a Razor error like it does during design time.

cc @dougbu @NTaylorMullen

[mkArtakMSFT]

Closing as no customers complained about this.