[NTG-context] texexec is very coy

[Sanjoy Mahajan]

When texexec doesn't find a program or a file, it doesn't complain
loudly.  In this output from a recent run

         extra program : dvipos z 

        total run time : 3 seconds

texexec went looking for 'dvipos'.  I didn't suspect any problem since I
didn't know that it wasn't installed (and hadn't heard of it until a few
hours ago).  An error message like this would be useful::

         extra program : dvipos z 
ERROR: Could not run 'dvipos'
        total run time : 3 seconds

Perhaps at the end of the run repeat all those error msgs (otherwise
they gets buried in all the output).

A similar message would be useful when the tex file itself doesn't exist:

$ texexec nosuchfile.tex

 TeXExec 5.4.3 - ConTeXt / PRAGMA ADE 1997-2005

fixing engine variable : pdfetex

        total run time : 1 seconds

Eventually I learnt that a run time of 1 second meant I'd mistyped the
file name, but a message would be useful (and more helpful than
prompting for the correct filename: I like Unix programs to not ask
questions).

-Sanjoy

`A society of sheep must in time beget a government of wolves.'
   - Bertrand de Jouvenal