Check and show an error message if we know user does not have enough ETH to send

AlphaWallet/Localization/en.lproj/Localizable.strings

@@ -244,6 +244,7 @@
 "a.claim.ticket.dollarCostLabel.title" = "EQUIVALENT IN USD";
 "a.send.receive.button.title" = "Send/Receive";
 "a.send.recipient.address.title" = "ADDRESS";
+"a.send.balance.insufficient" = "Insufficient balance";
 "accounts.confirm.delete.title" = "Are you sure you would like to delete this wallet?";
 "accounts.confirm.delete.message" = "Make sure you have backup of your wallet.";
 "accounts.confirm.delete.okTitle" = "Delete";

AlphaWallet/Localization/es.lproj/Localizable.strings

@@ -244,6 +244,7 @@
 "a.claim.ticket.dollarCostLabel.title" = "EQUIVALENT IN USD";
 "a.send.receive.button.title" = "Send/Receive";
 "a.send.recipient.address.title" = "ADDRESS";
+"a.send.balance.insufficient" = "Insufficient balance";
 "accounts.confirm.delete.title" = "Are you sure you would like to delete this wallet?";
 "accounts.confirm.delete.message" = "Make sure you have backup of your wallet.";
 "accounts.confirm.delete.okTitle" = "Delete";

AlphaWallet/Localization/zh-Hans.lproj/Localizable.strings

@@ -244,6 +244,7 @@
 "a.claim.ticket.dollarCostLabel.title" = "等价于美金";
 "a.send.receive.button.title" = "发送/接收";
 "a.send.recipient.address.title" = "地址";
+"a.send.balance.insufficient" = "Insufficient balance";
 "accounts.confirm.delete.title" = "你确认要删除这个钱包么?";
 "accounts.confirm.delete.message" = "请确认你已经做好钱包备份.";
 "accounts.confirm.delete.okTitle" = "删除";

AlphaWallet/Transfer/ViewControllers/SendViewController.swift

@@ -315,6 +315,10 @@ class SendViewController: UIViewController {
             return displayError(error: SendInputErrors.wrongInput)
         }
 
+        if case .ether = transferType, let balance = session.balance, balance.value < value {
+            return displayError(title: R.string.localizable.aSendBalanceInsufficient(), error: Errors.invalidAmount)
+        }
+
         let transaction = UnconfirmedTransaction(
                 transferType: transferType,
                 value: value,