Remove the `errors/errors.go` package and use core-lib

[alexec]

After reviewing the code, this package is almost always misused.

• Several error codes are available, but not used.
• BadRequest used liberally and incorrectly.
• Internal server error is pointless. All general errors are internal errors.

We should replace all calls with fmt.Errorf, os.ErrNotExist or apierr.XXX - depending of whether it is a file or API call.