Fixes qgis#21091

The issue was due to the /vsicurl/ not
being able to open a streaming endpoint.

“Things are not always what they seem; the first appearance
deceives many; [...]”

― Phaedrus

This reverts commit cf0d45e.

... and forward all app's message bar messages to
the dialog's message bar in case it is modal and
visible.

Also adds a question dialog when a /vsicurl/
add layer operation failed and offers a chance
to try the "normal" opening mode.

"loading ..." message and busy cursor