MAINT: Use exploding-ness of fx reader to determine currency-aware.

This produces a nicer error message for users if currency conversion is
requested, but not available.

Author: Scott Sanderson

zipline/pipeline/loaders/equity_pricing_loader.py

@@ -141,8 +141,8 @@ def load_adjusted_array(self, domain, columns, dates, sids, mask):
     @property
     def currency_aware(self):
         # Tell the pipeline engine that this loader supports currency
-        # conversion.
-        return True
+        # conversion if we have a non-dummy fx rates reader.
+        return not isinstance(self.fx_reader, ExplodingFXRateReader)
 
     def _inplace_currency_convert(self, columns, arrays, dates, sids):
         """