elf: Rename load_segments() and module failure

Rename this function to make it clear that it only reads loadable
segments. Also update the error for missing module to better match the
message emitted by Python.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Suggested-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>