Merge #13775: doc: Remove newlines from error message

Summary:
620361fce8 Fix accidental use of the addition assignment operator ("+="). Remove newlines from error message. (practicalswift)

Pull request description:

Fix accidental use of the addition assignment operator (`+=`).

_Note to reviewers:_ Perhaps the `\n`:s should be removed too?

Tree-SHA512: 4e8c2dfd6025d78ef9d60522297994829dacc447e6b6782e15c0bdd5dd2daa17ca9a8948bfa9a15be57d9286092356381d7e6747980303852d273eb0df0dd76b

Backport of Core PR13775
https://github.com/bitcoin/bitcoin/pull/13775/

Test Plan:

make check
test_runner.py
./bitcoind -blockdir=somenonsense

The last command should output the following error (with no newline at the end):

2019-12-18T21:50:03Z Error: Specified blocks directory "somenonsense" does not exist.
Error: Specified blocks directory "somenonsense" does not exist.

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4763