reduce likelihood of IndexError during DFU

In some cases `DfuDevice.control_msg` returns zero bytes. This might be
because the device didn't have enough time to process the previous
command. This commit adds a single retry after a delay. If the retry
fails too, an error message is printed that helps the user to recover
the ODrive.

Since this issue occurs rarely it's unclear if this commit really solves
the issue.

tools/odrive/dfuse/DfuDevice.py

@@ -2,6 +2,7 @@ import usb.util
 import time
 import fractions
 import array
+import time
 from odrive.dfuse.DfuState import DfuState
 
 DFU_REQUEST_SEND = 0x21
@@ -64,7 +65,20 @@ class DfuDevice:
         self.control_msg(DFU_REQUEST_SEND, DFU_CLRSTATUS, 0, None)
 
     def get_state(self):
-        return self.control_msg(DFU_REQUEST_RECEIVE, DFU_GETSTATE, 0, 1)[0]
+        msg = self.control_msg(DFU_REQUEST_RECEIVE, DFU_GETSTATE, 0, 1)
+
+        # Second chance after giving the device some time to breathe.
+        if len(msg) == 0:
+            time.sleep(0.5)
+            msg = self.control_msg(DFU_REQUEST_RECEIVE, DFU_GETSTATE, 0, 1)
+
+        if len(msg) == 0:
+            raise Exception("Could not get device state. Firmware upgrade will abort. "
+                            "Please try again. If odrivetool can't find the device "
+                            "anymore after this, follow the instructions in "
+                            "https://docs.odriverobotics.com/odrivetool#device-firmware-update "
+                            "(\"How to force DFU mode\").")
+        return msg[0]
 
     def abort(self):
         self.control_msg(DFU_REQUEST_RECEIVE, DFU_ABORT, 0, 0)