actions: Regularise SYNTAX_ERROR() statements

Get rid of the redundant "action: " prefix and make the
error messages match others.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
actions: Regularise SYNTAX_ERROR() statements

Get rid of the redundant "Action mode: " prefix and make the
error messages match others.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>