MAINT: Cleaned up sid checks and exception types

Removed unnecessary parens

Keeping NameError reserved for when locals or globals are not found.
Exception is what we use for the other sid checks, so now they are consistent.

zipline/finance/performance/position.py

@@ -88,8 +88,8 @@ class Position(object):
 
     # Returns the unused cash.
     def handle_split(self, split):
-        if (self.sid != split.sid):
-            raise NameError("updating split with the wrong sid!")
+        if self.sid != split.sid:
+            raise Exception("updating split with the wrong sid!")
 
         ratio = split.ratio
 
@@ -132,7 +132,7 @@ class Position(object):
         return return_cash
 
     def update(self, txn):
-        if(self.sid != txn.sid):
+        if self.sid != txn.sid:
             raise Exception('updating position with txn for a '
                             'different sid')