Suggestions to make this code more usable as a library

[Dan-Lightsource]

@wjasper Thanks for your work in putting together and sharing this comprehensive library. I found it to be a very useful reference to understand the workings of the underlying hardware interface on these USB devices.

I was initially hoping to use this code as a drop-in library, but encountered some obstacles in doing so. Here are some suggestions to consider for possible improvements in that regard:

• Many of the functions will invoke exit() on error. This is undesirable as it can bring down the whole application. Would prefer if it returned a defined set of error codes instead.
• Many of the functions will invoke printf() and friends in various places. This is undesirable for some applications which need to control what gets printed to stdout/stderr. An option to suppress verbose debug output and error messages would be preferable.
• return codes from libusb API calls are often not checked and/or not passed up to the application. This means that error conditions often cannot be detected and handled at application level.

[wjasper]

I've tried to remove as many printf and friends a possible. Some are unavoidable. Tried to return as many libusb API calls as possible. Thanks for the suggestion.