8674e74 Provide relevant error message if datadir is not writable. (murrayn)
Pull request description:
If the --datadir exists, but is not writable, the current error message on startup is 'Cannot obtain a lock on data directory foo. Bitcoin Core is probably already running.' This is misleading. I believe this PR addresses #11668, although the issue is not Windows-specific.
Tree-SHA512: 10cbbaea433072aee4fb3e8938a72073c7a5c841f7a7685c9e12549c322b2925c7d34bac254ac33021b23132bfc352c058712bc9542298cf86f8fd9757f528b2
Backport Core PR 12630
https://github.com/bitcoin/bitcoin/pull/12630/files
Completes T605
Depends on D2809