Give helpful message in case Matplotlib isn't working

openai#1

spinup/utils/plot.py

@@ -1,4 +1,15 @@
-import seaborn as sns
+import sys
+try:
+    import seaborn as sns
+except ImportError as e:
+    if "Python is not installed as a framework" in str(e):
+        print("It looks like you are using OSX and Anaconda setup, without "
+              "Matplotlib properly configured. \nIssuing this from the command "
+              "line should fix the problem:\n"
+              "echo \"backend: TkAgg\" >> ~/.matplotlib/matplotlibrc\n"
+              "For more info see "
+              "https://github.com/openai/spinningup/issues/1\n")
+        sys.exit(2)
 import pandas as pd
 import matplotlib.pyplot as plt
 import json