getrawchangeaddress should fail when keypool exhausted

An user on IRC reported an issue where `getrawchangeaddress`
keeps returning a single address when the keypool is exhausted.
In my opinion this is strange behaviour.

- Change CReserveKey to fail when running out of keys in the keypool.
- Make `getrawchangeaddress` return RPC_WALLET_KEYPOOL_RAN_OUT when
  unable to create an address.
- Add a Python RPC test for checking the keypool behaviour in combination
  with encrypted wallets.

src/rpcwallet.cpp

@@ -201,7 +201,7 @@ Value getrawchangeaddress(const Array& params, bool fHelp)
     CReserveKey reservekey(pwalletMain);
     CPubKey vchPubKey;
     if (!reservekey.GetReservedKey(vchPubKey))
-        throw JSONRPCError(RPC_WALLET_ERROR, "Error: Unable to obtain key for change");
+        throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
 
     reservekey.KeepKey();