Explicitly ensure that wallet is unlocked in `importprivkey`

This makes for a more useful error reply (fixes #957).

Rebased-from: 4175de73d954a02c9f57f603e63e0d688ddfeed2 0.8.x

src/rpcdump.cpp

@@ -39,6 +39,8 @@ Value importprivkey(const Array& params, bool fHelp)
             "importprivkey <litecoinprivkey> [label] [rescan=true]\n"
             "Adds a private key (as returned by dumpprivkey) to your wallet.");
 
+    EnsureWalletIsUnlocked();
+
     string strSecret = params[0].get_str();
     string strLabel = "";
     if (params.size() > 1)