Allow the load command to fail

When a device is selected from the list, its configuration is loaded
into the tmpcfg buffer.  When this failed due to there being no
configuration in the device, the ensuing exception prevented the device
from being configured by the GUI.

Now we explicitly silence the KeyError when loading configuration.  The
tmpcfg is still valid, so it can be read and manipulated just as always.

pd-buddy-gtk.py

         window = self.builder.get_object("pdb-window")
         try:
             with pdbuddy.Sink(self.serial_port) as pdbs:
-                pdbs.load()
+                try:
+                    pdbs.load()
+                except KeyError:
+                    # If there's no configuration, we don't want to fail
+                    pass
                 self.cfg = pdbs.get_tmpcfg()
         except OSError as e:
             comms_error_dialog(window, e)