Get rid of the aerc.PushError(" " + $string) idiom

The individual callers should not be responsible for padding

commands/compose/attach.go

@@ -34,13 +34,13 @@ func (Attach) Execute(aerc *widgets.Aerc, args []string) error {
 
 	path, err := homedir.Expand(path)
 	if err != nil {
-		aerc.PushError(" " + err.Error())
+		aerc.PushError(err.Error())
 		return err
 	}
 
 	pathinfo, err := os.Stat(path)
 	if err != nil {
-		aerc.PushError(" " + err.Error())
+		aerc.PushError(err.Error())
 		return err
 	} else if pathinfo.IsDir() {
 		aerc.PushError("Attachment must be a file, not a directory")

commands/compose/postpone.go

@@ -63,7 +63,7 @@ func (Postpone) Execute(aerc *widgets.Aerc, args []string) error {
 	go func() {
 		errStr := <-errChan
 		if errStr != "" {
-			aerc.PushError(" " + errStr)
+			aerc.PushError(errStr)
 			return
 		}
 
@@ -90,7 +90,7 @@ func (Postpone) Execute(aerc *widgets.Aerc, args []string) error {
 				r.Close()
 				composer.Close()
 			case *types.Error:
-				aerc.PushError(" " + msg.Error.Error())
+				aerc.PushError(msg.Error.Error())
 				r.Close()
 				composer.Close()
 			}