Make the common error messages more consistent

There are still several cases where the user could see an error not
prefixed with "pq: ", but there's no real reason why we couldn't fix
these instances.

copy.go

@@ -8,10 +8,10 @@ import (
 )
 
 var (
-	errCopyInClosed               = errors.New("copyin statement has already been closed")
-	errBinaryCopyNotSupported     = errors.New("only text format supported for COPY")
-	errCopyToNotSupported         = errors.New("COPY TO is not supported")
-	errCopyNotSupportedOutsideTxn = errors.New("COPY is only allowed inside a transaction")
+	errCopyInClosed               = errors.New("pq: copyin statement has already been closed")
+	errBinaryCopyNotSupported     = errors.New("pq: only text format supported for COPY")
+	errCopyToNotSupported         = errors.New("pq: COPY TO is not supported")
+	errCopyNotSupportedOutsideTxn = errors.New("pq: COPY is only allowed inside a transaction")
 )
 
 // CopyIn creates a COPY FROM statement which can be prepared with

notify.go

@@ -339,8 +339,8 @@ func (l *ListenerConn) Err() error {
 
 var errListenerClosed = errors.New("pq: Listener has been closed")
 
-var ErrChannelAlreadyOpen = errors.New("channel is already open")
-var ErrChannelNotOpen = errors.New("channel is not open")
+var ErrChannelAlreadyOpen = errors.New("pq: channel is already open")
+var ErrChannelNotOpen = errors.New("pq: channel is not open")
 
 type ListenerEventType int