If SPL fails to boot, try to provide an error code to indicate what is
wrong. For example, if a uclass is missing, this can return -EPFNOSUPPORT
(-96) which provides useful information.

Add a helper for accessing the image-loader name so we can drop the use
of #ifdefs in this code.

Put this feature behind a CONFIG_SHOW_ERRORS option to avoid increasing
the code size.

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