I just removed the battery, put it back in and the phone would boot up normally again without going into the firmware upgrade error screen.
I expected it to boot back into the leaked 5.0 that was on it before the "unsuccessful" flash, it took a while for it to optimized all my 313 apps but it finally booted up.
And: tadaa - checking device information showed me that in fact the flash was a full success!
I'm on N9005XXUGBOA1 now.
All apps and settings untouched.
And the best thing: finally my smartwatch bluetooth re-connection issues have gone, the gear 2 connected instantly after entering the SIM pin and unlocking the screen!
Lindex = Happy!