Add error codes to Warnings

[joj]

There is a LogCodedWarning method in Utilities (https://github.com/xamarin/xamarin-android/blob/master/src/Xamarin.Android.Build.Tasks/Utilities/MSBuildExtensions.cs#L119) that is never used, as far as I can tell. We need warnings to provide proper error codes, so VS can show them in the correct column in the error list and we can react automatically to known errors that may be fixed easily.

As part of this, we need to make sure that LogCodedError is used instead of LogError (or that LogError is always used with errorCode as a parameter), for the same reason.

VS bug #601721

[bholmes]

Jon found 2 instances of LogCodedWarning via grep. We do need more.

[jonpryor]

What we should also do is document these codes. We should add a new Documentation/guides/messages directory, which contains one .md file per warning/error code, which contains information about the message.

For example, Documentation/guides/messages/xa4212.md would contain information about error code XA4212, added in PR#797.