ENH: Well formed exception for any value passed to OrderStyle

This commit adds support for arbitrary objects in addition to NaN
and infinity values. The object well be returned in string format
as part of the error message.

tests/test_execution_styles.py

@@ -62,7 +62,19 @@ class ExecutionStyleTestCase(TestCase):
         for delta in range(1, 10)
     ]
 
-    INVALID_PRICES = [(-1,), (-1.0,), (0 - epsilon,), (float('nan'),)]
+    class ArbitraryObject():
+        def __str__(self):
+            return """This should yield a bad order error when
+            passed as a stop or limit price."""
+
+    INVALID_PRICES = [
+        (-1,),
+        (-1.0,),
+        (0 - epsilon,),
+        (float('nan'),),
+        (float('inf'),),
+        (ArbitraryObject(),),
+    ]
 
     def setUp(self):
         setup_logger(self)