[GUI][Bug] Notify transaction creation failure reason

[random-zebra]

Triggered by #1619

• first commit fixes a bug where the WalletModel::message signal was not connected to pwidget, therefore the message with strFailReason -fired by prepareTransaction:
  

  PIVX/src/qt/walletmodel.cpp

  Lines 526 to 533 in f295ee4

  	if (!fCreated) {
  	if ((total + nFeeRequired) > nBalance) {
  	return SendCoinsReturn(AmountWithFeeExceedsBalance);
  	}
  	Q_EMIT message(tr("Send Coins"), QString::fromStdString(strFailReason),
  	CClientUIInterface::MSG_ERROR);
  	return TransactionCreationFailed;
  	}

  
  was never displayed, and the user was presented only with the generic "Transaction creation failed!" -fired by ProcessSendCoinsReturnAndInform:
  

  PIVX/src/qt/pivx/guitransactionsutils.cpp

  Lines 74 to 79 in f295ee4

  	void ProcessSendCoinsReturnAndInform(PWidget* parent, const WalletModel::SendCoinsReturn& sendCoinsReturn, WalletModel* walletModel, const QString& msgArg, bool fPrepare)
  	{
  	CClientUIInterface::MessageBoxFlags informType;
  	QString informMsg = ProcessSendCoinsReturn(parent, sendCoinsReturn, walletModel, informType, msgArg, fPrepare);
  	if (!informMsg.isEmpty()) parent->emitMessage(parent->translate("Send Coins"), informMsg, informType, 0);
  	}

• second commit addresses the fact that, with the previous commit, the user is now presented with two consecutive dialogs: "Transaction too big" and "Transaction creation failed!", and merges them into one single message (which is already followed by a SnackBar with "Cannot create transaction").

• third commit adds a clearer message when strFailReason is "Transaction too big" (which might be frequent e.g. sweeping a big number of masternode rewards or dust outputs). Reviewers are encouraged to offer better wording.

Closes #1619

[ambassador000]

Functionality tested, working as intended.

Just minor suggestion: coin control -> Coin Control to keep consistency.

[furszy]

Concept ACK and code looking good, good fix.

Left a comment for the future.

[Fuzzbawls]

utACK 05981ea

[furszy]

utACK 05981ea