Problem

In practice, most of these I/O errors need to be displayed to the end user through logs or prompts. Showing an error code to the user doesn't make much sense:

local _, code = fs.remove("file", Url("/foo"))
ya.dbg("Failed to remove `/foo`, error code: " .. code)

which prints:

Failed to remove `/foo`, error code: 2

Goal

This PR changes the type of I/O errors from a number to a new Error type.

The Error type implements the __tostring() and __concat() meta method, allowing it to generate more human-readable error messages:

local _, err = fs.remove("file", Url("/foo"))
ya.dbg("Failed to remove `/foo`, error: " .. err)

which prints:

Failed to remove `/foo`, error: No such file or directory (os error 2)

In certain cases where the raw error code is needed, it can be accessed via the .code property of the Error:

local _, err = fs.remove("file", Url("/foo"))
ya.dbg("Failed to remove `/foo`, error code: " .. err.code)

which prints:

Failed to remove `/foo`, error code: 2

Breaking changes

After this PR, the second return value of the following functions will change from a numeric error code to the new Error type:

fs API:

• fs.cha()
• fs.write()
• fs.remove()
• fs.read_dir()
• fs.unique_name()

Command API:

• Command:spawn()
• Command:output()
• Command:status()

Child API:

• Child:write_all()
• Child:flush()
• Child:wait()
• Child:wait_with_output()
• Child:start_kill()