Give `http://melta.no` troubleshooting help link when plugin fails, with CTA to Slack in web copy

Spec (edited by AJ):

When Meltano fails, we’ll print a standard error message:

[Error description text.]

To contact the Meltano team and access troubleshooting help and resources, visit http://melta.no

Orig. issue description (Douwe)

When meltano invoke or meltano run fails, a user is more likely to manage to solve the issue and stick with Meltano if they join Slack than if they don’t, as evidenced by the number of people who responded to the survey saying the docs weren’t always helpful, but the Slack community was. I imagine a good amount of people that try us out and churn would not have if they had known about Slack. That also lets us reach out to them to learn more about their use case and experience, even if they do end up churning.

So I think we should recommend joining Slack in the product wherever that makes sense.

@tayloramurphy Wdyt? Seems like an easy change with potentially large impact.

[tayloramurphy]

I’ve just updated the melta.no forwarding to go to https://docs.meltano.com/guide/troubleshooting

[tayloramurphy]

@aaronsteers that spec looks good to me 👍