driver/rawnetworkinterfacedriver: raise when tcpdump fails to start listening

If tcpdump exits cleanly without emitting "tcpdump: listening on"
(possibly due to the message being changed), readline() is called on a
closed stderr stream, resulting in confusing exceptions. Fix that by
raising a proper exception in that case.

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

labgrid/driver/rawnetworkinterfacedriver.py

@@ -237,6 +237,8 @@ def start_record(self, filename, *, count=None, timeout=None):
                     e.stderr = stderr
                     raise
 
+                raise ExecutionError(f"tcpdump exited before emitting 'tcpdump: listening on': {stderr}")
+
             if line.startswith(b"tcpdump: listening on"):
                 break