Error handling
==============

Functions that can fail must have a way of telling the caller that something
went wrong and preferably what went wrong. The simplest and cleanest way of 
indicating an error is a return code. Error codes are defined in the <errno.h>
header alongside some helpfull macros.

Error codes have identifiers like these: ENOMEM, ENOENT, EALIGN, ...
They are defined as positive values, however all functions should return a
negative integer on error, so it would look something like 'return -EFAULT;'.
Also theres SUCCESS which is just 0. You can use it, but don't need to.

If you are returning a pointer/address, the highest address is almost certainly
unused, so returning '(void*)-EINVAL', should work just fine. This is quite an
eyesore, so there are three macros for working with these "errno-in-pointer"s.
IS_ERR(ptr) just returns 1 if the pointer is an errno. Otherwise 0.
ERR_PTR(err) does that void* cast mentioned above so it would be ERR_PTR(-EINVAL).
PTR_ERR(ptr) is the inverse of ERR_PTR(), as it converts a errno-in-pointer to
an errno value. PTR_ERR(ERR_PTR(-EIO)) == -EIO


Here are some usage examples:
	int pag_init(void) {
		if (...) return -ENOMEM; // Not enough memory
		
		void* paddr = pmm_alloc();
		if (IS_ERR(paddr)) return PTR_ERR(paddr); // Pass the error down

		return SUCCESS;
	}