Provide way to return error to caller instead of calling `abort()` on failed assertions

[adsr]

Aborting is fine for debug but in production it is usually undesirable to crash. For example, on failed malloc, an error code or NULL can be returned to the caller.

[hnes]

Aborting is fine for debug but in production it is usually undesirable to crash.

On some occasions "let it crash" idea is very convenient and powerful. For example, in the case of libaco, the memory for creating coroutine is usually not the bottleneck of resource usage. But I do admit that there still exist some occasion that needs to handle such resource-draining.

Thanks a lot for this proposal, @adsr 👍