* src/stringutils.cc (filename_to_utf8_fallback): On failure,
don't fall back to the locale encoding anymore but straight to
plain ASCII.  Thus from now on, this function will never throw
for errors of type ConvertError::ILLEGAL_SEQUENCE.  Also cover
the new G_FILENAME_ENCODING variable in the warning message.

* src/filetree.cc (FileTree::find_files): Remove the explicit
handling of Glib::ConvertError.