lib/util: Some style conversion

I saw a few instances of `glnx_set_error_from_errno() + return FALSE`,
and fixed them and did a bit of style conversion.

Closes: #895
Approved by: jlebon

src/libotutil/ot-gio-utils.c

@@ -100,10 +100,7 @@ ot_gfile_ensure_unlinked (GFile         *path,
   if (unlink (gs_file_get_path_cached (path)) != 0)
     {
       if (errno != ENOENT)
-        {
-          glnx_set_error_from_errno (error);
-          return FALSE;
-        }
+        return glnx_throw_errno_prefix (error, "unlink(%s)", gs_file_get_path_cached (path));
     }
   return TRUE;
 }