Take care of base "HealthCheck" class (Follow-up from "Fix ClickHouse healthcheck")

The following discussion from !14187 should be addressed:

• @ebaque started a discussion: (+1 comment)

  This is where this trailing space comes from: https://gitlab.com/gitlab-org/customers-gitlab-com/-/blob/536d14dc867767e272168c794a66f586c92f7298/lib/health_check/custom.rb#L6

  I'm not sure why it's there in the first place, but I feel like the original writer was concerned about concatenation of health check error message.

  To me this is worth addressing in a follow-up MR (I can take care of this, but this is not urgent though)