Add error handling for OSC_URL

[weex]

The script will crash only when a payment comes in if the OSC_URL is incorrect. We should put the call to bpn.php in a try statement and have it log an error instead of crashing.

For extra credit, we can add a handler to the bpn.php that enables us to check that we're able to access it and have that check done right when monitor.py is first run even when no payments have come in. That check should end the script with a helpful error message.