Implement error/warning infrastructure in the type system

[MichalStrehovsky]

Good warnings and error reporting are of great help to tool developers or general developers alike.

The warning system should not be a burden in environments where warnings are undesired/unactionable - it should get out of the way without impacting perf. At the same time we want the warning system be flexible enough to enable PEVerify-like scenarios (dotnet/coreclr/#295) for the type system in the future.

The error system should be based on standardized exceptions that provide context with programmatic access (e.g. having an object-typed Pertinent property that points to a type system object associated with the exception).

[MichalStrehovsky]

This has been fixed in e9f8056.