bootstd: Tidy up reporting of errors

In a few cases the error handling is not quite right. Make sure we
return the actual error in distro_efi_read_bootflow_file() rather than
-EINVAL. Return -IO when a file cannot be read. Also show the error name
if available.

This does not change operation, but does make it easier to diagnose
problems.

Signed-off-by: Simon Glass <sjg@chromium.org>