Show error and exit on shader/texture file open failure

[satoshinm]

When craft cannot load shader or texture files (for example, if launched outside of its resource directory), it would crash with a segmentation fault:

native-build $ ./craft 
error 78: failed to open file for reading
Segmentation fault: 11

This PR fixes this crash by exiting with an error, as well as printing the file which failed to load:

native-build $ ./craft
load_png_texture textures/texture.png failed, error 78: failed to open file for reading

[fogleman]

Thank you, I appreciate small PRs!