Return error response based on caught exception from postData

[mkienenb]

I'm resubmitting this pull request.

The original PR #89 was cancelled without fixing the general issue (caught exceptions are logged but return null responses) because the symptom was fixed (xml parsing exception).

Summary of the general issue:

When an exception occurs during an ANetApiResponse.get(), the exception is sent to the logger, and the method returns null instead of a valid ANetApiResponse. The API eventually returns a null response to the caller with no information about what occurred.

This PR returns an error ANetApiResponse containing the information from the exception instead of null if an exception is caught, which allows the caller to do something with the exception.

[Note: the updated CONTRIBUTING.md guidelines refer to the future branch, which no longer exists.]