Phlume iOS - Error handling and unit testing.

Summary:
Added some error handling to the API, for reporting errors more properly rather than just ignoring them.

Added some unit and integration tests to help ensure continued functionality as more diffs are made.

Fixed issue where headers weren't being included when the project was built.

Fixed an issue where getting a file by file id would return a dictionary object instead of a file object.

Test Plan: Run unit tests.

Reviewers: ablackwood

Reviewed By: ablackwood

Differential Revision: https://devportal.imobile3.com/D11754