backend: retry datasource requests for trivial network errors

[hagen1778]

See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6712

In the issue above, request sent by Grafana failed because of EOF error. There could be many reasons why plugin faced the EOF error, but the sensible thing to do would be to retry such trivial network errors.

For the reference, other VM components like vmalert already retry requests for such errors.