Add PySnmpError.cause attribute

To convey parent exception information on re-raise

CHANGES.txt

@@ -1,5 +1,5 @@
 
-Revision 5.0.0, released 2018-07-??
+Revision 5.0.0, released 2018-09-??
 -----------------------------------
 
 - SNMPv3 crypto operations that require external dependencies
@@ -27,9 +27,10 @@ Revision 5.0.0, released 2018-07-??
         methods. The goal is to let MIB objects access/modify whatever
         custom Python objects they need while being called back.
 
-Revision 4.4.5, released 2018-07-XX
+Revision 4.4.5, released 2018-08-XX
 -----------------------------------
 
+- Added PySnmpError.cause attribute holding parent exception tuple
 - Fixed zero SNMPv3 boots/time values put in SNMPv3 TRAP messages
 - Fixed broken InetAddressType rendering caused by a pyasn1 regression
 - Fixed typo in RFC1158 module

pysnmp/error.py

@@ -5,6 +5,10 @@
 # License: http://snmplabs.com/pysnmp/license.html
 #
 
+import sys
+
 
 class PySnmpError(Exception):
-    pass
+    def __init__(self, message):
+        self.cause = sys.exc_info()
+        Exception.__init__(self, '%s, caused by %s: %s' % (message, self.cause[0], self.cause[1]))