Propogate additional compilation data from RoslynCompilationService

* Update CompilationFailedException to include path of file being compiled
* Pass in path being compiled to Rolsyn.
* Adding doc comments for compilation pieces

Fixes #869
We never have a cs file, the cshtml is codegened in memory to produce C# content. In System.Web, these files had to be saved to disk since it was the only way to invoke `csc.exe` to produce a type.