dlsym: Used without proper error handling.

[cheako]

In atw_vulkan.c, at least, dlsym is used as though it's guarantied the symbol being looked up exists. This may not be correct as it assumes that the Vulkan Loader was built and installed correctly.

To be correct dlsym should be wrapped in dlerror calls and given the usage here it may make sense to put this handling into a function that reports error and exits on symbol not found.