[m-rev.] for post-commit review: further improve parse_pragma.m

[Julien Fischer]

Hi Zoltan,

On Thu, 16 Mar 2017, Zoltan Somogyi wrote:

> For review by Julien.

> Improve error messages for malformed pragmas further.
> 
> compiler/parse_pragma.m:
>     Instead of reporting that a pragma in a file has the "wrong number of
>     arguments", explicitly say what the *right* number of arguments are.
>
>     Fix a missing colon.
>
>     Make a message conform to our 'expected X, got Y' pattern.
>
>     Future proof some code against future changes that may make ContextPieces
>     empty.

What future changes do you have in mind?

The diff is fine.

Julien.