Fix 2 small issues in sporks module (bitcoin#2133)

* Fix a crash when someone tries to update sporks without specifying `sporkkey`

Before:
Crashes with `Assertion failed: (fValid), function GetPubKey, file key.cpp, line 156.`

After:
Fails and prints rpc help for `spork`

* Silence logging in CSporkMessage::CheckSignature() a bit

CMessageSigner::VerifyMessage() failure is not really an error
if CHashSigner::VerifyHash() below passes with no issues.
No need to log this.

src/spork.cpp

@@ -241,6 +241,11 @@ uint256 CSporkMessage::GetSignatureHash() const
 
 bool CSporkMessage::Sign(const CKey& key)
 {
+    if (!key.IsValid()) {
+        LogPrintf("CSporkMessage::Sign -- signing key is not valid\n");
+        return false;
+    }
+
     CKeyID pubKeyId = key.GetPubKey().GetID();
     std::string strError = "";
 
@@ -290,7 +295,6 @@ bool CSporkMessage::CheckSignature(const CKeyID& pubKeyId) const
         std::string strMessage = boost::lexical_cast<std::string>(nSporkID) + boost::lexical_cast<std::string>(nValue) + boost::lexical_cast<std::string>(nTimeSigned);
 
         if (!CMessageSigner::VerifyMessage(pubKeyId, vchSig, strMessage, strError)){
-            LogPrintf("CSporkMessage::CheckSignature -- VerifyMessage() failed, error: %s\n", strError);
             // Note: unlike for other messages we have to check for new format even with SPORK_6_NEW_SIGS
             // inactive because SPORK_6_NEW_SIGS default is OFF and it is not the first spork to sync
             // (and even if it would, spork order can't be guaranteed anyway).