Fix string concatenation to os.path.join and add exception case
Fix string concatenation to os.path.join and add exception case

Summary:
Backport of core PR11291
https://github.com/bitcoin/bitcoin/pull/11291/files

Test Plan:
  make check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3427

cculianu/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,cculianu/bitcoin-abc,cculianu/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,Bitcoin-ABC/bitcoin-abc,ftrader-bitcoinabc/bitcoin-abc,cculianu/bitcoin-abc