CiviCRM upgrade from 5.12 to 5.29 - failure
Welcome to SE!  There's lots of useful advice from Luke in his answer but I'd start by looking at the CiviCRM log file.
That error message is a bit misleading.  It's not random luck as to whether an upgrade succeeds so restoring to the previous state and just trying the same upgrade code is very unlikely to produce a better result.  The upgrade probably failed because of a mismatch between what the code was expecting to find in your system, and what it actually found.  Until you track down and correct that problem reloading and repeating won't help.
You can often fix the problem and continue the upgrade without reloading a backup.
If you look at the log file you should see the various upgrade steps logged there.  What was the last successful upgrade step?  What messages do you get after that?
Update
The log file shows:
Row size too large. The maximum row size for the used table type, not counting BLOBs, is 8126. This includes storage overhead, check the manual. You have to change some columns to TEXT or BLOBs
Updated table to row_format=dynamic and then reran the upgrade.