database/sql: support ErrSkip in Tx.Exec
database/sql: support ErrSkip in Tx.Exec

If the database driver supports the Execer interface but returns
ErrSkip, calling Exec on a transaction was returning the error instead
of using the slow path.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5654044

Committer: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang