Return results even if some query fails

[Maros112358]

Is your feature request related to a problem? Please describe.
In sql_exporter 0.12.1, when using multiple targets and jobs, if one of the requests failed when scraping /metrics, other metrics were returned successfully. In sql_exporter 0.12.2, if one query fails, whole request returns 500: Internal server error.

Describe the solution you'd like
I prefered the behaviour in 0.12.1. Since this was probably changed intentionally in the last release, I would like the option to return succesfully queried metrics, even if some of them fail.

Describe alternatives you've considered
Using 0.12.1, but I need some bug fixes introduced in 0.12.2. I also removed queries usually returning errors, but this is not good long term solution for me.

Additional context
I am monitoring multiple databases, and sometimes some queries fail, affecting data from other databses. I would like to avoid this.

[burningalchemist]

Fair enough, we should be able to watch error_scrape metric instead, which will be delivered soon. 👍

[Maros112358]

Tried out. Looks good to me. Thank you 👍