spdx30: Improve os.walk() handling

spdx30: Improve os.walk() handling

There have been errors seen when assembling root file system SPDX
documents where they will references files that don't exist in the
package SPDX.

The speculation is that this is caused by os.walk() ignoring errors when
walking, causing files to be omitted. Improve the code by adding an
error handler to os.walk() to report errors when they occur.

In addition, sort the files and directories while walking to ensure
consistent ordering of the file SPDX IDs.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>