In a transaction, on a Stmt.Query error, it was possible for a
connection to be added to a db's freelist twice. Should use
the local releaseConn function instead.

Thanks to Gwenael Treguier for the failing test.

Also in this CL: propagate driver errors through releaseConn
into *DB.putConn, which conditionally ignores the freelist
addition if the driver signaled ErrBadConn, introduced in a
previous CL.

R=golang-dev, gary.burd
CC=golang-dev
https://golang.org/cl/5798049