Added extra check for verifying data source

Author: koshtinikhilesh

pandas_datareader/data.py

@@ -303,6 +303,16 @@ def DataReader(name, data_source=None, start=None, end=None,
     ff = DataReader("6_Portfolios_2x3", "famafrench")
     ff = DataReader("F-F_ST_Reversal_Factor", "famafrench")
     """
+    expected_source = ["yahoo", "google", "iex", "iex-tops", "iex-last", "iex-last", "bankofcanada",
+                       "stooq", "iex-book", "enigma", "fred", "famafrench", "oecd", "eurostat", "nasdaq",
+                       "quandl", "moex", "morningstar", 'robinhood', 'tiingo', "yahoo-actions",
+                       "yahoo-dividends", "av-forex", "av-daily", "av-daily-adjusted", "av-weekly",
+                       "av-weekly-adjusted", "av-monthly", "av-monthly-adjusted"]
+
+    if data_source not in expected_source:
+        msg = "data_source=%r is not implemented" % data_source
+        raise NotImplementedError(msg)
+
     if data_source == "yahoo":
         return YahooDailyReader(symbols=name, start=start, end=end,
                                 adjust_price=False, chunksize=25,