Such errors are difficult to spot because most of the time the
corresponding markers are displayed only in the right gutter, not in
the editor (though, sometimes a single character is highlighted for
some reason).
For an empty expression fragment the whole its content range is
highlighted (from "{" to either "!", ":" or "}").
For the first unclosed expression fragment the range from "{" to the
end of the containing string node is highlighted.
In case of a missing conversion character the character "!" itself is
highlighted.
