Clarified ImportError

theano/d3viz/formatting.py

@@ -51,7 +51,8 @@ class PyDotFormatter(object):
     def __init__(self, compact=True):
         """Construct PyDotFormatter object."""
         if not pydot_imported:
-            raise ImportError('Failed to import pydot. Please install pydot!')
+            raise ImportError('Failed to import pydot. You must install pydot'
+                              ' and graphviz for `PyDotFormatter` to work.')
 
         self.compact = compact
         self.node_colors = {'input': 'limegreen',