BZ 6701 - Improve error messages (updated-patch)

[Frank Ch. Eigler]

Hi -

> [...] Please have a look at the updated patch, and any further
> suggestions are more than welcome.

Thanks for going once more around.  I am afraid I noticed one other
largish issue.  That is the multiple-instance hard-coded use of of
session.user_file->input_contents to feed the source printer.  That's
not right, since errors may occur associated with any stapfile.  All
you would have to go on in general is what was given in the
semantic_error object (token pointers).

So what's probably needed here is an additional field in the token
object that points to the stapfile (or perhaps even to the file
contents directly, if identical std::string sharing works properly).

- FChE