Error handling

Useful references:
http://gamedev.stackexchange.com/questions/46424/try-catch-or-ifs-for-error-handling-in-c
http://bitsquid.blogspot.ca/2012/01/sensible-error-handling-part-1.html

- Adopt an uniform method of handling errors (instead of returning bools at some places and 0xFFFFFFFF at other places...)
- If keeping the 0xFFFFFFF, use UINT32_MAX instead
- For shaders it might be a bigger problem if an error occurs, because the program will simply not work properly (in contrast with the textures which is not a big problem for the app itself if they fail)