Use qInfo() if no error occurs

Summary:

qWarning() is used to report warnings and recoverable errors in your
application.
qInfo() is used for informational messages (since Qt 5.5).

This is a backport of Core PR16263

Test Plan:
Those messages can be printed with this command:
src/qt/bitcoin-qt -printtoconsole -debug=qt | grep "GUI:"

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8372