reserve the error value from VSL_Write() and friends

include/vapi/vsl.h

@@ -339,6 +339,7 @@ VSL_Check(const struct VSL_cursor *c, const struct VSLC_ptr *ptr);
 	 */
 
 enum vsl_status {
+	vsl_e_write	= -5,	// Error from VSL_Write etc.
 	vsl_e_io	= -4,	// I/O read error - see errno
 	vsl_e_overrun	= -3,	// Overrun
 	vsl_e_abandon	= -2,	// Remote abandoned or closed