main: Print specific errors for invalid arguments
main: Print specific errors for invalid arguments

This avoids the large usage message obscuring the actual errors from
parsing options.  Print usage only if an option is invalid.

Signed-off-by: Amitay Isaacs <a505e7c038746dbeb3edcd8d146dbd515432801f@ozlabs.org>

open-power/pdbg,open-power/pdbg,open-power/pdbg