mncc_sock.py: throw / raise exceptions from MnccSocket

Calling sys.exit() from MnccSocket's constructor could lead to
a situation when the main thread becomes "dead", while its child
threads are still working. Let's throw the exceptions towards
the caller, so it would be possible to handle them there.

Change-Id: I485eb86c69e74693eeae89723de27e376585f1da

mncc_sock.py

@@ -10,7 +10,6 @@
 # option, any later version.
 
 import socket
-import sys
 import os
 import mncc
 import ctypes
@@ -122,11 +121,7 @@ class MnccSocket(MnccSocketBase):
         super(MnccSocketBase, self).__init__()
         self.sock = socket.socket(socket.AF_UNIX, socket.SOCK_SEQPACKET)
         print('connecting to %s' % address)
-        try:
-            self.sock.connect(address)
-        except socket.error as errmsg:
-            sys.stderr.write("%s\n" % errmsg)
-            sys.exit(1)
+        self.sock.connect(address)
 
         # Check the HELLO message
         self.check_hello()
@@ -137,15 +132,13 @@ class MnccSocket(MnccSocketBase):
 
         # Match expected message type
         if msg.msg_type != mncc.MNCC_SOCKET_HELLO:
-            sys.stderr.write('Received an unknown (!= MNCC_SOCKET_HELLO) '
+            raise AssertionError('Received an unknown (!= MNCC_SOCKET_HELLO) '
                 'message: %s\n' % msg)
-            sys.exit(1)
 
         # Match expected protocol version
         if msg.version != mncc.MNCC_SOCK_VERSION:
-            sys.stderr.write('MNCC protocol version mismatch '
+            raise AssertionError('MNCC protocol version mismatch '
                 '(0x%04x vs 0x%04x)\n' % (msg.version, mncc.MNCC_SOCK_VERSION))
-            sys.exit(1)
 
         # Match expected message sizes / offsets
         if (msg.mncc_size != ctypes.sizeof(mncc.gsm_mncc) or
@@ -154,8 +147,7 @@ class MnccSocket(MnccSocketBase):
             msg.signal_offset != mncc.gsm_mncc.signal.offset or
             msg.emergency_offset != mncc.gsm_mncc.emergency.offset or
             msg.lchan_type_offset != mncc.gsm_mncc.lchan_type.offset):
-                sys.stderr.write('MNCC message alignment mismatch\n')
-                sys.exit(1)
+                raise AssertionError('MNCC message alignment mismatch\n')
 
         print('Received %s' % msg)