Ignore garbage packets in synchronization state

The MCU might still be running user code and toggling the UART pins
when stcgal is started. Just ignore all garbage we see in this stage
until we get a valid package.

stcgal.py

@@ -1218,10 +1218,17 @@ class Stc12Protocol:
 
         print("Waiting for MCU, please cycle power: ", end="")
         sys.stdout.flush()
-        self.pulse()
+
+        # send sync, and wait for MCU response
+        # ignore errors until we see a valid response
+        status_packet = None
+        while not status_packet:
+            try:
+                self.pulse()
+                status_packet = self.get_status_packet()
+            except (RuntimeError, serial.SerialException): pass
         print("done")
 
-        status_packet = self.get_status_packet()
         self.initialize_status(status_packet)
         self.initialize_model()
         self.initialize_options(status_packet)
@@ -1646,7 +1653,7 @@ class StcGal:
             print("Communication error: %s" % e, file=sys.stderr)
             self.protocol.disconnect()
             return 1
-        except serial.serialutil.SerialException as e:
+        except serial.SerialException as e:
             print("Serial communication error: %s" % e, file=sys.stderr)
             return 1
 
@@ -1669,7 +1676,7 @@ class StcGal:
             print("interrupted")
             self.protocol.disconnect()
             return 2
-        except serial.serialutil.SerialException as e:
+        except serial.SerialException as e:
             print("Serial communication error: %s" % e, file=sys.stderr)
             return 1