insns.pl: better error messages, handle no-operand instructions better
insns.pl: better error messages, handle no-operand instructions better

Better error messages, and allow "void" instructions to omit the
operand colon.

techkey/nasm,turingstudio/nasm,projedi/nasm,Distrotech/nasm,Distrotech/nasm,techkey/nasm,techkey/nasm,Distrotech/nasm,techkey/nasm,turingstudio/nasm,projedi/nasm,Distrotech/nasm,projedi/nasm,letolabs/nasm,turingstudio/nasm,letolabs/nasm,projedi/nasm,letolabs/nasm,turingstudio/nasm,techkey/nasm,projedi/nasm