Call TraceQueryEnd during Exec errors

[cbandy]

We should make every effort to call TraceQueryEnd after calling TraceQueryStart. Conn.Query handles this case but not Conn.Exec:

pgx/conn.go

Lines 756 to 761 in 7fd58a7

	if err := c.deallocateInvalidatedCachedStatements(ctx); err != nil {
	if c.queryTracer != nil {
	c.queryTracer.TraceQueryEnd(ctx, c, TraceQueryEndData{Err: err})
	}
	return &baseRows{err: err, closed: true}, err
	}