Map getlogin_r()'s ERANGE to ERROR_GEN_FAILURE

ERROR_INSUFFICIENT_BUFFER should never occur here and would be a
misleading error for the user.

src/impl/getusername.cpp

@@ -96,7 +96,7 @@ BOOL GetUserNameW(WCHAR_T* lpBuffer, LPDWORD lpnSize)
 			errno = ERROR_NO_ASSOCIATION;
 			break;
 		case ERANGE:
-			errno = ERROR_INSUFFICIENT_BUFFER;
+			errno = ERROR_GEN_FAILURE;
 			break;
 		case ENOENT:
 			errno = ERROR_NO_SUCH_USER;