Convert JuliaError.__str__ to python error (on branch v1)

[dpinol]

Strangely, this works fine from PythonCall.jl unit tests... :

try:
    import juliacall
    from juliacall import JuliaError

    juliacall.Main.error()
except JuliaError as e:
    print(e)

..but causes this on my project where I'm migrating to v1:

    jl.fail()
../../../.julia/packages/PythonCall/xZbVY/src/JlWrap/any.jl:479: in __call__
    return self._jl_callmethod($(pyjl_methodnum(pyjlany_call)), args, kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   juliacall.JuliaError: <exception str() failed>

During handling of the above exception, another exception occurred:
test/julia/test_jl.py:25: in test_pyjlvalues
    print(e)
E   TypeError: __str__ returned non-string (type Jl)

However, I guess it makes sense to convert to python string.