Merge #12630: Provide useful error message if datadir is not writable.

Summary:
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

Test Plan: make check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2810