Irc: simplify runloop error catching, adding RuntimeError and SystemExit (closes #438)

socket.error is aliased to OSError since Python 3.3, and ConnectionError is actually a subclass of OSError.
So, it makes more sense to just catch the more generic type here.

Also, make ProtocolError derive from RuntimeError instead of Exception.

(cherry picked from commit 397df48efd)

classes.py

@@ -27,7 +27,7 @@ from .log import *
 
 ### Exceptions
 
-class ProtocolError(Exception):
+class ProtocolError(RuntimeError):
     pass
 
 ### Internal classes (users, servers, channels)
@@ -312,7 +312,11 @@ class Irc():
                               'trying to set up this connection. Please check'
                               ' your configuration file and try again.',
                               self.name)
-            except (socket.error, ProtocolError, ConnectionError) as e:
+            # Note: socket.error, ConnectionError, IOError, etc. are included in OSError since Python 3.3,
+            # so we don't need to explicitly catch them here.
+            # We also catch SystemExit here as a way to abort out connection threads properly, and stop the
+            # IRC connection from freezing instead.
+            except (OSError, RuntimeError, SystemExit) as e:
                 # self.run() or the protocol module it called raised an
                 # exception, meaning we've disconnected!
                 log.error('(%s) Disconnected from IRC: %s: %s',