What is the history of this iPhone?
Was it already at 10.3?
Possibly a DFU restore will get it fixed but with loss of data. And my belief is with error 26 your data partition is bloated and unrecoverable anyway.
EDIT August 4, 2017: following the comments on the OP's question, here's an important affirmation:
At this date, you cannot get data from a device that fails to update and throws errors 26.
Whoever tells you otherwise is trying for easy non-refundable diagnosis money or don't know what they are talking about, just assuming it may be some random failure they can fix to recover data..
Don't send your devices to anyone who claims to get data from an Error 26 device unless they offer a no-fix-no-fee solution.
Don't pay for any software or any person making this claim until you have proof that they have your data recovered or you are certain to get a 100% refund in case of failure.