[m-dev.] diff: prog_io.m error detection fixes

[Andrew Bromage]

G'day.

Fergus Henderson wrote:

> Fix some bugs where the compiler was silently ignoring certain errors.
> 
> compiler/prog_io.m:
> 	Check for some possible errors in function declarations that
> 	were previously being silently ignored.  Specifically, we
> 	now catch cases where a function type declaration includes
> 	modes for some but not all of its arguments or return value,
> 	or where it includes a determinism but no modes.

This makes tests/hard_coded/ho_func_reg.m fail.  There was a syntax
error there to begin with and this found it.

Cheers,
Andrew Bromage