Added RpcConnectionError exception class.

NoFreeLabelError and FallbackModeEnabledError are now extending this new
class.

sjrpc/core/exceptions.py

@@ -25,7 +25,14 @@ class SocketRpcError(Exception):
     '''
 
 
-class NoFreeLabelError(Exception):
+class RpcConnectionError(Exception):
+
+    '''
+    Base class for RpcConnection errors.
+    '''
+
+
+class NoFreeLabelError(RpcConnectionError):
 
     '''
     Exception raised when no more free labels are available for protocol
@@ -33,7 +40,7 @@ class NoFreeLabelError(Exception):
     '''
 
 
-class FallbackModeEnabledError(Exception):
+class FallbackModeEnabledError(RpcConnectionError):
 
     '''
     Exception raised when a feature which is not compatible with fallback mode