Review error handling

[jackc]

Errors are part of the package interface. Review to consider if errors are being exposed that shouldn't be or if there are new types or interfaces that should be.

The current error structure uses golang.org/x/xerrors to use the proposed Go 1.13 error system. PgError is the only exposed error type. There are several sentinel error values. At the moment all error checking must be performed with Is or As because of possible wrapping.

golang/go#29934
golang/go#32405

[jackc]

Added some internally strong types for errors and added helper functions for checking if it is safe to retry after an error and if an error was caused by a timeout.

138254d