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/miner.cpp

@@ -467,7 +467,10 @@ void static BitcoinMiner(CWallet *pwallet)
 
             auto_ptr<CBlockTemplate> pblocktemplate(CreateNewBlockWithKey(reservekey));
             if (!pblocktemplate.get())
+            {
+                LogPrintf("Error in BitcoinMiner: Keypool ran out, please call keypoolrefill before restarting the mining thread\n");
                 return;
+            }
             CBlock *pblock = &pblocktemplate->block;
             IncrementExtraNonce(pblock, pindexPrev, nExtraNonce);
 

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();
 

src/wallet.cpp

@@ -2010,11 +2010,7 @@ bool CReserveKey::GetReservedKey(CPubKey& pubkey)
         if (nIndex != -1)
             vchPubKey = keypool.vchPubKey;
         else {
-            if (pwallet->vchDefaultKey.IsValid()) {
-                LogPrintf("CReserveKey::GetReservedKey(): Warning: Using default key instead of a new key, top up your keypool!");
-                vchPubKey = pwallet->vchDefaultKey;
-            } else
-                return false;
+            return false;
         }
     }
     assert(vchPubKey.IsValid());