When detecting compiler type, fail silently instead of propagating OSError.
When detecting compiler type, fail silently instead of propagating OSError.

When we want to detect the compiler type, it is possible the passed compiler 
does not exists, so we catch OSerror exception launched by popen_wrapper, and return
a failure. This way, the user will get an UNkwnownCompiler exception instead, which 
is much clearer. 

cournape/numscons,cournape/numscons,cournape/numscons