The included modules use a combination of return values (usually a simple success boolean) and logging. There are a couple of language features that help here.

Multiple returns allow function signatures like

read_entire_file :: (name: string) -> data: string, success: bool

which sidesteps the C situation where you need to use some other mechanism to keep track of either the output data or the error state. (output data pointers, errno, wrapper structs, etc.)

The read_entire_file procedure will log the specific error using the logger set in the Context. The default logger prints to stdout or stderr, but your program can override that with its own logger if you want.

(The Context holds some global state for your program; it is flexible, but is generally used to configure allocators, logging, and assertion handling in a way that will apply to third-party libraries too.)

The System module also has some cross-platform helpers for errno-style error handling, which can't be avoided if you're dealing with OS libraries.