Clean up get() calls, remove commented out code

@Ignat99 PTAL.

As suggested by @dionyziz at https://github.com/OpenBazaar/OpenBazaar/pull/658#issuecomment-56653243 I created a PR out of my own code-review comments.
OK, but I try to avoid too much indentation, so I'm going to slightly change your suggestion to this:
```Python
try:
    contract_body = json.loads(u"%s" % contract['contract_body'])
except (KeyError, ValueError) as e:
    self.log.error("Problem loading the contract body JSON: %s", e.message)
    continue

try:
    item_price = max(0, contract_body['Contract']['item_price'])
except KeyError:
    item_price = 0
try:
    shipping_price = max(0, contract_body['Contract']['shipping_price'])
except KeyError:
     shipping_price = 0

my_contracts.append(...)
```
I think that's what we actually want.  We want the contract to be skipped (see the ```continue```) in the loop if there is an error.