RPC dumpwallet and z_exportwallet updated to no longer allow

overwriting an existing file.

qa/rpc-tests/walletbackup.py

@@ -34,6 +34,7 @@ and confirm again balances are correct.
 """
 
 from test_framework.test_framework import BitcoinTestFramework
+from test_framework.authproxy import JSONRPCException
 from test_framework.util import assert_equal, initialize_chain_clean, \
     start_nodes, start_node, connect_nodes, stop_node, \
     sync_blocks, sync_mempools
@@ -141,6 +142,14 @@ class WalletBackupTest(BitcoinTestFramework):
         self.nodes[2].backupwallet("walletbak")
         self.nodes[2].dumpwallet("walletdump")
 
+        # Verify dumpwallet cannot overwrite an existing file
+        try:
+            self.nodes[2].dumpwallet("walletdump")
+            assert(False)
+        except JSONRPCException as e:
+            errorString = e.error['message']
+            assert("Cannot overwrite existing file" in errorString)
+
         logging.info("More transactions")
         for i in range(5):
             self.do_one_round()