Add Fluent::TextParser::ParserError to improve error handling

For better error handling.
Plugin can catch TextParser specific error.
Which is better?

`error = 'undefined method 'to_i' for []:Array', error_class = 'NoMethodError'`

or

`error = 'NoMethodError: undefined method 'to_i' for []:Array'`

We can use `Exception#cause` method since Ruby 2.1. So adding nested exception info to latest exception is not needed in the future.