[m-rev.] for review: improve messages for some mode errors

[Julien Fischer]

On Fri, 24 Jan 2020, Zoltan Somogyi wrote:

> For review by anyone.

> Improve error messages for some mode errors.
> 
> compiler/mode_errors.m:
>     There are two improvements.
>
>     The first and much the more important is that when a call does not match
>     any of the declared modes of the called predicate or function, we now
>     print the names and insts of any arguments whose insts, by themselves,
>     do not match any of the callee's modes. Instead of reporting "here are
>     all the arguments, here are all their insts, find out for yourself
>     which one is wrong", the compiler now tells the programmer explicitly
>     which one is wrong in almost all cases. (Not all. If e.g. two args <A, B>
>     have insts <free, free>, but the callee has two modes, <in, out> and
>     <out, in>, then the compiler won't report either arg, since neither arg
>     is wrong by itself.)
>
>     The second improvement is that when the verbosity settings ask us
>     to print all possible reasons why we can't schedule a conjunction,
>     instead of our default approach of printing the only the first,
>     we now put a blank line between the different reasons, for visual
>     separation.

That looks fine; did you intend this for the release branch as well?

Julien.