util_libc: open_readonly shoud return a Result

Signed-off-by: Joe Richey <joerichey@google.com>

src/util_libc.rs

@@ -129,15 +129,15 @@ cfg_if! {
 }
 
 // SAFETY: path must be null terminated, FD must be manually closed.
-pub unsafe fn open_readonly(path: &str) -> Option<libc::c_int> {
+pub unsafe fn open_readonly(path: &str) -> Result<libc::c_int, Error> {
     debug_assert!(path.as_bytes().last() == Some(&0));
     let fd = open(path.as_ptr() as *mut _, libc::O_RDONLY | libc::O_CLOEXEC);
     if fd < 0 {
-        return None;
+        return Err(last_os_error());
     }
     // O_CLOEXEC works on all Unix targets except for older Linux kernels (pre
     // 2.6.23), so we also use an ioctl to make sure FD_CLOEXEC is set.
     #[cfg(target_os = "linux")]
     libc::ioctl(fd, libc::FIOCLEX);
-    Some(fd)
+    Ok(fd)
 }