Improve exception handling

[sharkodlak]

Exception can be enhanced by passing causing exception as $previous when ApiException is created. Now it's impossible to get API response body from ApiException.

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Expected Behavior

I'd like to get API response body from ApiException $e.

try {
...
} catch (ApiException $e)
(string) $e->getPrevious()->getResponse()->getBody();
}

Current Behavior

It's impossible to call $e->getPrevious()->... because previous exception is't stored.

Steps to Reproduce

1. Set editMode: $client->editMode(true);
2. Call $client->getStories($options)->getBody(); Now ApiException is thrown, because client is unauthorized.
3. Try to obtain serious information what's going on from ApiException $e.

[roberto-butti]

Thank you @sharkodlak for the valuable feedback.
Let me check it and come back to you with more information.