Add hierarchy logic to custom exceptions.

bfxapi/exceptions.py

@@ -0,0 +1,18 @@
+__all__ = [
+    "BfxBaseException",
+    "LabelerSerializerException"
+]
+
+class BfxBaseException(Exception):
+    """
+    Base class for every custom exception in bfxapi/rest/exceptions.py and bfxapi/websocket/exceptions.py.
+    """
+
+    pass
+
+class LabelerSerializerException(BfxBaseException):
+    """
+    This exception indicates an error thrown by the _Serializer class in bfxapi/labeler.py.
+    """
+
+    pass