Avoid crash when g_thread_http was never started

Summary:
Avoid a crash during shutdown when the init sequence failed for some reason

fa83b39ff3ae3fbad93df002915c0e5f99c104a9

init: Remove confusing and redundant InitError

The "A fatal internal error occurred, see debug.log for details" is
redundant because init.cpp will already show an InitError with a better
error message as well as the hint to check the debug.log

fa12a37b27f0570a551b8c103ea6537ee4a8e399

test: Replace inline-comments with logs, pep8 formatting

faf45d1f1f997c316fc4c611a23c4456533eefe9

http: Avoid crash when g_thread_http was never started

g_thread_http can not be joined when it is not joinable. Avoid crashing
the node by adding the required check and add a test.

This is a backport of Core PR19006

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9230