Additional UserException categories for internal errors

The UserException class in Drill is the way that Drill passes error conditions from the server to the client. UserException groups errors into a number of categories as defined in the protobuf definitions.

As we enhance error handling in operators, it becomes clear that some errors do not fit into the existing categories. In particular:

• Execution errors: problems related to run-time issues where the cause is not known, or the error does not fit into an existing category. This might occur, say, if a temporary file fills, a spill file is deleted or other odd-ball cases.
• Internal error: error that indicates failure of Drill code, invariants or the like. Internal errors are generally not something wrong with the user's data, query or environment, but rather something wrong with Drill itself. For example, all IllegalStateException, AssertionError or IllegalArgumentException errors would map to an internal error.
• Unspecified error: for cases where Drill code caches an exception from a deeper layer, but we don't know enough about the exception to render a useful categorization. Most often used in cases of plugins that don't do adequate error handing themselves.