Add cmd_process_error() to report and process errors

U-Boot now uses errors defined in include/errno.h which are negative
integers. Commands which fail need to report the error and return 1
to indicate failure. Add this functionality in cmd_process_error().

For now this merely reports the error number. It would be possible
also to produce a helpful error message by storing the error strings
in U-Boot.

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