Add error handling to Greeter tutorial (#774)

* Add error handling to Greeter tutorial

While trying to get up and running with contract deployment through geth, I spent multiple hours trying to debug the `greeterFactory.new(...)` command in this tutorial without realizing that I was getting an "Error: exceeds block gas limit" error. This change makes it so that the console will log if contract deployment fails.

* Use early return for error handling.

views/content/greeter.md

@@ -101,16 +101,17 @@ You have now compiled your code and made it available to Geth.  Now you need to
 	var _greeting = "Hello World!"
 
     var greeter = greeterFactory.new(_greeting,{from:eth.accounts[0],data:greeterCompiled,gas:47000000}, function(e, contract){
-        if(!e) {
-
-          if(!contract.address) {
-            console.log("Contract transaction send: TransactionHash: " + contract.transactionHash + " waiting to be mined...");
+        if(e) {
+          console.error(e); // If something goes wrong, at least we'll know.
+          return;
+        }
 
-          } else {
-            console.log("Contract mined! Address: " + contract.address);
-            console.log(contract);
-          }
+        if(!contract.address) {
+          console.log("Contract transaction send: TransactionHash: " + contract.transactionHash + " waiting to be mined...");
 
+        } else {
+          console.log("Contract mined! Address: " + contract.address);
+          console.log(contract);
         }
     })