Die on failed /incbin/ seeks

Failing to open an input file, with /include/ or /incbin/ is treated as
immediately fatal inside srcfile_relative_open().  However, filing to
seek() to the requested offset in an /incbin/ is not.  This is a bit oddly
inconsistent, and leaves us with a strange case that's awkward to deal with
down the line.

So, get rid of it and have failed seeks on an /incbin/ be immediately
fatal.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>