Increase the error message buffer to 1024 as recommended in the manual page (Thanks Jon Atack)
Deduplicate code and error checks by making sure `s` stays `nullptr` in case of error. Return "Unknown error" instead of an empty string in this case.
Some uses of non-threadsafe `strerror` have snuck into the code since they were removed in #4152. Add a wrapper `SysErrorString` for thread-safe strerror alternatives and replace all uses of `strerror` with this.
