Error handling when sending commands

Before now, if an error occurred when sending a command, it passed
silently.  This is a Bad Thing™, so it's been fixed.  Now if there's an
error, an error dialog appears with some information about what
happened.  If the error occurred on the Sink page, the program goes back
to the device list after the error dialog is closed.

pd-buddy-gtk.py

 from gi.repository import Gtk, Gio, GObject, GLib
 
 
-def pdb_send_message(sp, message):
+def pdb_send_message(window, sp, message):
     """Send a message over the serial port and return the response"""
-    # Open the serial port
-    # FIXME handle exceptions
-    sp = serial.Serial(sp.device, baudrate=115200, timeout=0.01)
+    try:
+        # Open the serial port
+        sp = serial.Serial(sp.device, baudrate=115200, timeout=0.01)
 
-    sp.write(bytes(message, 'utf-8') + b'\r\n')
-    sp.flush()
-    answer = sp.readlines()
+        sp.write(bytes(message, 'utf-8') + b'\r\n')
+        sp.flush()
+        answer = sp.readlines()
 
-    sp.close()
+        sp.close()
 
-    # Remove the echoed command and prompt
-    answer = answer[1:-1]
-    return answer
+        # Remove the echoed command and prompt
+        answer = answer[1:-1]
+        return answer
+    except OSError as e:
+        dialog = Gtk.MessageDialog(window, 0, Gtk.MessageType.ERROR,
+                Gtk.ButtonsType.CLOSE, "Error communicating with device")
+        dialog.format_secondary_text(e.strerror)
+        dialog.run()
+
+        dialog.destroy()
+        raise
 
 
 class ListRowModel(GObject.GObject):
         self.show_all()
 
     def on_identify_clicked(self, button):
-        pdb_send_message(self.model.serport, 'identify')
+        window = self.get_toplevel()
+        try:
+            pdb_send_message(window, self.model.serport, 'identify')
+        except:
+            return
 
 
 class Handler:
 
         self.serial_port = serport
 
-        pdb_send_message(self.serial_port, 'load')
-        tmpcfg = pdb_send_message(self.serial_port, 'get_tmpcfg')
+        window = self.builder.get_object("pdb-window")
+        try:
+            pdb_send_message(window, self.serial_port, 'load')
+            tmpcfg = pdb_send_message(window, self.serial_port, 'get_tmpcfg')
+        except:
+            return
 
         # Get voltage and current from device and load them into the GUI
         for line in tmpcfg:
         st.set_visible_child(sink)
 
     def on_header_sink_back_clicked(self, data):
+        self.serial_port = None
+
         # Show the Select page
         hst = self.builder.get_object("header-stack")
         hselect = self.builder.get_object("header-select")
         self._store_device_settings()
         self._set_save_button_visibility()
 
-        pdb_send_message(self.serial_port, 'write')
+        window = self.builder.get_object("pdb-window")
+        try:
+            pdb_send_message(window, self.serial_port, 'write')
+        except:
+            self.on_header_sink_back_clicked(None)
 
     def _store_device_settings(self):
         """Store the settings that were loaded from the device"""
     def on_voltage_combobox_changed(self, combo):
         self._set_save_button_visibility()
 
-        pdb_send_message(self.serial_port,
-                         'set_v {}'.format(int(combo.get_active_text())*1000))
+        window = self.builder.get_object("pdb-window")
+        try:
+            pdb_send_message(window, self.serial_port,
+                             'set_v {}'.format(int(combo.get_active_text())*1000))
+        except:
+            self.on_header_sink_back_clicked(None)
 
     def on_current_spinbutton_changed(self, spin):
         self._set_save_button_visibility()
 
-        pdb_send_message(self.serial_port,
-                         'set_i {}'.format(int(spin.get_value()*1000)))
+        window = self.builder.get_object("pdb-window")
+        try:
+            pdb_send_message(window, self.serial_port,
+                             'set_i {}'.format(int(spin.get_value()*1000)))
+        except:
+            self.on_header_sink_back_clicked(None)
 
 class Application(Gtk.Application):