No such dam shall be erected to the injury of any mill lawfully existing, either above or below it on the same stream; nor to the injury of any mill site on the same stream on which a mill or milldam shall have been lawfully erected and used or is in the process of erection, unless the right to maintain a mill on such last-mentioned site shall have been lost or defeated by abandonment or otherwise; nor to the injury of any such mill site which has been occupied as such by the owner thereof, if such owner, within a reasonable time after commencing such occupation, completes and puts in operation a mill for the working of which the water of such stream shall be applied.