Fix PySnmpError implementation

This is a follow up fix to make PySnmpError properly
overriding base Exception call signature

pysnmp/error.py

@@ -9,6 +9,15 @@ import sys
 
 
 class PySnmpError(Exception):
-    def __init__(self, message):
+    def __init__(self, *args):
+        msg = args and str(args[0]) or ''
+
         self.cause = sys.exc_info()
-        Exception.__init__(self, '%s, caused by %s: %s' % (message, self.cause[0], self.cause[1]))
+
+        if self.cause[0]:
+            msg += 'caused by %s: %s' % (self.cause[0], self.cause[1])
+
+        if msg:
+            args = (msg,) + args[1:]
+
+        Exception.__init__(self, *args)