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);