Bug fix: Catch all incorrect responses

Catch all exceptions when decoding a JSON response from Exponea.

Catch erroneous response

Currently, the SDK was applying

result = json.loads(response.text)

without catching any errors. Exponea sometimes returns a non-JSON format response which causes this to crash.
I've added logging the response from Exponea and raising APIException.

Refactoring code

Refactored code based on #3.