caltechlibrary/dataset: Python 3.6 module error_message() function

Description

Added a error_message() function to Python module so you can check for the last error recorded in the Go runtime. e.g. ```python record = dataset.read(my_collection, "1023") e_msg = dataset.error_message() if e_msg != "": handle_error(e_msg) ```

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Dataset is a command line tool for working with JSON (object) documents on local disc, in an S3 bucket or on Google Sheets/Cloud Storage

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