Make Julia exceptions wrapped in Python Exceptions more informative

I have added a backtrace to the description of exceptions coming from Julia
that are wrapped in Python exceptions.

This makes it much easier to find the source of errors in Julia code being
called from Python functions.

src/exception.jl

@@ -132,7 +132,7 @@ be called via Python C-API, it tries to not throw at all cost.
 function showerror_string(e::T) where {T}
     try
         io = IOBuffer()
-        showerror(io, e)
+        showerror(io, e, catch_backtrace())
         return String(take!(io))
     catch
         try