Avoid overly long error messages (and prevent crashes)

If you open a path in Nautilus that is long enough to trigger G_IO_ERROR_FILENAME_TOO_LONG, you’ll notice a crash with this joyful message (at least in a Wayland session):

(nautilus:12167): Gdk-CRITICAL **: gdkdisplay-wayland.c:1363: Unable to create Cairo image surface: invalid value (typically too big) for the size of the input (surface, pattern, etc.)

This comes from create_message_dialog() in eel-stock-dialogs.c trying to cram the whole error message into the secondary text label. There’s nothing really smart that we can do apart from truncating error messages at a reasonable character count (100).