Improved error handling

Hi,
I’ve seen that recently there was some changes in error handling, however, current apporach isn’t working properly:

With your code:
```
Venice::Receipt.verify!(Rails.root.join("spec","receipt").read)
RuntimeError: This receipt is a sandbox receipt, but it was sent to the production service for verification.
	from /Users/krzyzak/projects/venice/lib/venice/client.rb:60:in `verify!'
	from /Users/krzyzak/projects/venice/lib/venice/receipt.rb:92:in `verify!'
```
With my code:
```
Venice::Receipt.verify!(Rails.root.join("spec","receipt").read)

#<Venice::Receipt:0x007fe67bf9ca90 @quantity=1, @product_id="com.mindmobapp.download", @transaction_id="1000000046178817", @purchase_date=Mon, 30 Apr 2012 15:05:55 +0000, @app_item_id=nil, @version_external_identifier=nil, @bid="com.mindmobapp.MindMob", @bvrs="20120427", @original=#<Venice::Receipt:0x007fe67bf9c220 @product_id=nil, @transaction_id="1000000046178817", @purchase_date=Mon, 30 Apr 2012 15:05:55 +0000, @app_item_id=nil, @version_external_identifier=nil, @bid=nil, @bvrs=nil>
```

There are some failing tests (but originally were) – I don’t have time now to fix them, but I’m sure that I’ll try to fix them within few days ;)