[segment] Add "Requested memory not available" error message

prep_segment() can sometimes fail because an image requests memory
that is already in use by gPXE.  This will happen if
e.g. undionly.kpxe is used to boot memtest86; the memtest86 image is
an old-format kernel that needs to be loaded at 9000:0000, but this
area of memory may well already be in use by the underlying PXE stack.

Add a human-friendly error message, so that the cause is more
immediately visible.

src/hci/strerror.c

@@ -115,5 +115,6 @@ struct errortab common_errors[] __errortab = {
 	{ ENOSPC, "No space left on device" },
 	{ ENOTSUP, "Not supported" },
 	{ EPERM, "Operation not permitted" },
+	{ ERANGE, "Out of range" },
 	{ ETIMEDOUT, "Connection timed out" },
 };