OK I had the same Error 50 in iTunes for an iPhone 5s updating software. My phone could not start, at all.
None of the Apple official support pages worked, neither did the other posts.
So I booked into the local Genius bar and went in. Like probably you, my iPhone 5s is out of warranty and has seen better days.
I was ready with my story. How many hours I had danced the dance. Different USB cables, recovery mode, itunes boot up, quit, mac boot up, reboot etc etc.
Bottom line. The update to 9.2 had bricked my phone and....but I didnt need my story or anything.
The friendly Genius fella booted up iTunes, uploaded and applied software, then iTunes got to the Update Firmware section where it failed everytime for me and....no problem. Phone completed updated and is back up and running.
It appears for reasons even the Apple Genius could not explain, the error 50 is isolated to remote updates. In other words, an update done anywhere but at Apple.
I had used wifi, the Genius used wifi, the Genius used his cable. Nothing else different. Go figure.