Make InfluxDB::Errors StandardErrors again

**WIP** expect rebasing!

As discussed in https://github.com/influxdb/influxdb-ruby/issues/40 inheriting custom exceptions from `Exception` is bad practice and poses some unexpected behavior to gem users.

I made `InfluxDB::Error` inherit from `StandardError` again.

`Client#connect_with_retry` now also only retries exceptions raised by `net/http` so that `Client#query` used with blocks can also benefit from being used with `Client#connect_with_retry`.

## TODOs

- [ ] add specs for `Client#connect_with_retry`'s retry logic