Support for exceptions thrown from ExecuteUpdate and ExecuteDelete

[MarkusG]

Because the project uses a SaveChangesInterceptor, exceptions thrown by calls to ExecuteUpdate or ExecuteDelete or their async variants will not be intercepted. Supporting these is simple enough: just add a DbCommandInterceptor overriding CommandFailed(Async). However, the current structure of the project doesn't allow this to be done easily, as each provider provides its own interceptor, and the DatabaseError enum is a member of the ExceptionProcessorInterceptor class.

I propose each provider has its own exception processor, but there be only one SaveChangesInterceptor and DbCommandInterceptor for project. The interceptors can take the provider's exception processor in their constructors when instantiated in UseExceptionProcessor().

For now, I'm working on a PR to add support using the existing pattern with an extra interceptor for each provider, but I'm creating an issue for discussion.