Exit gracefully with an error code whenever backend exits

ethermess.py

@@ -511,7 +511,7 @@ def eventloop(proc):
 
 			elif fd == proc.stdout.fileno() and event & select.POLLHUP:
 				print('--- Backend exited')
-				running = False
+				sys.exit(1)
 
 			elif fd == unbuf_stdin.fileno() and event & select.POLLIN:
 				data = unbuf_stdin.read(1024)
@@ -557,7 +557,11 @@ def main():
 	writeall(proc.stdin, bytes([own_status.value, len(encoded)]) + encoded)
 
 	# Read our MAC
-	mac = readall(proc.stdout, 6)
+	try:
+		mac = readall(proc.stdout, 6)
+	except ReadallError:
+		print('--- Backend exited')
+		sys.exit(1)
 
 	print('--- MAC: %s' % format_mac(mac))