Auto merge of #27995 - nagisa:windows-error-message, r=alexcrichton
Auto merge of #27995 - nagisa:windows-error-message, r=alexcrichton

According to https://msdn.microsoft.com/en-us/library/windows/desktop/ms679351(v=vs.85).aspx:

> If the function succeeds, the return value is the number of TCHARs stored in the output buffer,
> excluding the terminating null character.

_**Completely untested**_… since I have no Windows machine or anything of a sort to test this on.

r? @aturon 

aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust