logilab-common #253516 [graph] clearer exception when dot is not installed [validation pending]

currently we get:

 File "/usr/lib64/python2.7/subprocess.py", line 1308, in _execute_child
   raise child_exception
OSError: [Errno 2] No such file or directory
prioritynormal
typebug
done in0.63.0
load0.250
load left0.000
closed by#62386f95f6a5 [graph] More explicit error message if dot is not found (closes #253516)
patch[graph] More explicit error message if dot is not found (closes #253516) [applied]