Make Loader emit a signal when loading a dictionary fails; this way
Ui parts can receive that signal and display an appropriate message box.
At the moment DictionaryCombobox, listens to that signal, (the combobox
is used in configWidget, which is embedded in configuration dialogs of
apps using Sonnet, e.g. Kate (both in the config dialog and in the
spellcheck bar), Calligrawords).