[python] Cryptic error messages when creating arrays of disallowed types

[mojaveazure]

When trying to create a sparse or dense array of a disallowed type, the only message is "invalid pyarrow type string"; this should be updated to something more informative explaining that the type is disallowed by the SOMA spec

Example:

>>> shape = (200, 100)
>>> pyarr = numpy.array([""] * numpy.prod(shape), dtype=numpy.str_)
>>> pyarr.shape = shape
>>> atype = pyarrow.infer_type(pyarr[0])
>>> atype
DataType(string)
>>> arr = tiledbsoma.DenseNDArray.create(r.uri, type=atype, shape=pyarr.shape)
TypeError: Invalid pyarrow type string