don't continue pydotprint if we can't import pydot.

theano/printing.py

@@ -369,6 +369,7 @@ def pydotprint(fct, outfile=os.path.join(config.compiledir,'theano.pydotprint.pn
         import pydot as pd
     except:
         print "failed to import pydot. Yous must install pydot for this function to work."
+        return
     
     g=pd.Dot()
     var_str={}