What version of the Python interpreter are you seeing this with? String exceptions have been deprecated, but they were not supposed to be removed until Python 3.0 I thought (according to PEP 317). bx-python is currently only available for Python 2.
I'm using Python 2.6.5:
Python 2.6.5 (r265:79063, Apr 13 2010, 15:07:42)
[GCC 4.0.1 (Apple Inc. build 5484)] on darwin
I'm pretty sure the intent of these string exceptions was to get a
message to the user. So, if none of them are trapped by calling
routines, an easy short term fix would be to change them to asserts,
i.e. "raise s" becomes "assert False, s".