[Mesa-dev] Improve scons error reporting

[Jorge Villaseñor]

Hi I have tried to build mesa using scons with drivers=llvmpipe  and
llvm=yes options in a box without llvm and the only thing I got was a
python backtrace.

I think it would be helpful to catch that exception an prompt a user
friendly "llvm can't be found".

I guess the whole build system have the same problem spread everywhere.

PS: I'm not providing a patch because I'm just starting to read the code.

-- 
Masca

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