I tried the iPhone delete option - but - it was greyed out for me and the drag also was disabled, so I devised another method. This also deletes any files that the iPhone can't see but which result in the 9905 import error when I tried to import them normally.
METHOD:- Step One - clean out Iphone.
I renamed my iphoto library so that it could not be seen by the 'Photos' app - and then started the Photos app. It asks for me to find or create a new library. I created a new one. Then, I downloaded all my photos and videos with the DELETE from iphone option ticked. Downloading to a new, empty library fixed it for me.
Step Two - get all data in to mast library
I wanted these photo in my old library - so.. I renamed the new library - so that 'Photos' wouldn't see it, and restored the name of my master library so that 'Photos' could see it again.
Then, I right clicked on the new - renamed and thus invisible to Photos library and viewed the package contents. I dug down the nested directories until I found the one with the newly downloaded data (photos and videos and bursts and slow-mos and imported it to my master library.
I did the import by selecting 'Import' from the 'Photos' file menu at the top of the screen and dragged the directory in to the select window. It took some time, but no duplicates were loaded and all missing data was imported. I then trashed the renamed temporary library.
Job done.
Took me ages to work that one out.
Hope it helps !