Error message should indicate lightgbm installation, not scikitlearn (#218)

Author: vinitra

onnxmltools/convert/main.py

@@ -43,7 +43,7 @@ def convert_libsvm(model, name=None, initial_types=None, doc_string='', target_o
 def convert_lightgbm(model, name=None, initial_types=None, doc_string='', target_opset=None,
                      targeted_onnx=onnx.__version__, custom_conversion_functions=None, custom_shape_calculators=None):
     if not utils.lightgbm_installed():
-        raise RuntimeError('scikit-learn is not installed. Please install lightgbm to use this feature.')
+        raise RuntimeError('lightgbm is not installed. Please install lightgbm to use this feature.')
 
     from .lightgbm.convert import convert
     return convert(model, name, initial_types, doc_string, target_opset, targeted_onnx,