DFU mode: improve programming robustness.

dfu.get_status() might return usb.core.USBError with errno == EPIPE,
when called in quick succession. This is the case when called from
dfu.block_on_state(). The next call usually works.

This change tries to catch this specific error and just re-tries in case
it happens.

Without this change, the error (and programming abort) might get
undetected, as click catches IOerror/EPIPE silently. This has
significant potential to brick a device.

solo/dfu.py

@@ -7,6 +7,7 @@
 # http://opensource.org/licenses/MIT>, at your option. This file may not be
 # copied, modified, or distributed except according to those terms.
 
+import errno
 import struct
 import time
 
@@ -210,7 +211,10 @@ def block_on_state(self, state):
         s = self.get_status()
         while s.state == state:
             time.sleep(s.timeout / 1000.0)
-            s = self.get_status()
+            try:
+                s = self.get_status()
+            except usb.core.USBError as e:
+                if e.errno != errno.EPIPE: raise
 
     def read_option_bytes(self,):
         ptr = 0x1FFF7800  # option byte address for STM32l432