Fix marking JavaScript lines that threw an exception

Since EditorManager::openedEditors doesn't exactly do what one would
expect, it was only marking a single editor even if there were multiple
editors on the same document.

Change-Id: Iae579c2cfd8455de2783d39fa69944fe92dfc60a
Reviewed-by: Aurindam Jana <aurindam.jana@digia.com>