standard error messages

[Kolarik, Tony]

I'd like to have an easy way to parse a log file for any errors encountered
when building with cons.  

Here's a diff against 2.1.2 that attempts to categorize exit conditions and
report them accordingly.
I kept it simple and didn't do a lot of analysis of what could have caused
the errors and ended up with:
        cons usage_error: 
        cons build_error:
        cons error: 

I also added:
        cons warning: 

Now I can grep for: 
        /cons [a-z_]*error:/
(and no doubt someone has a better regexp for me too :-) 
 -- Tony K.

 <<err_msgs.diff>>

err_msgs.diff
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