Improve DType Exception Handling for CSV Read/Write

[stanbrub]

DTypes are documented for use with Deephaven CSV. However, all data types are not supported. The read_csv operation may throw a cryptic exception while the write_csv operation doesn't throw an exception but happily write out something to CSV that makes no sense.

It is not clear from either the exceptions or the documentation which column types are supported.

Examples:

• read_csv while passing in dht.BigDecimal results in a KeyError
• write_csv writes an array column without an exception but writes square brackets and identity hash, which is not useful.
• write_csv write Vector summary [a,b,c...], which is not useful either
• write_csv writes the value a value that read_csv won't read

Resources:

• https://deephaven.io/core/docs/reference/data-import-export/CSV/readCsv/
• https://deephaven.io/core/docs/reference/python/deephaven-python-types/