User-friendly message when callable schema fails in both Update and Create views

[corradio]

An exception in a the callable schema of an integration won't be caught and will cause a 500 when user creates/duplicates/edits a metric. This can happen for numerous reasons (for example an HTTP call which is not authorized anymore).

For now, only invalid_grant errors are handled, and they are only handled in the UpdateView (not CreateView).

We should:

• have a unified way of handling exceptions between create and update views
• show a friendly user message both when the metric can be re-authorised (e.g. invalidgrant) and when the error is generic
• mention the issue is with the integration and link to github

[corradio]

Note: this is still a problem even after 0e4b46b with the CreateView (when creating a metric from scratch), due to the fact that the error happens when rendering the form, which happens after the context has been processed (in the create of a new metric creation there is not context).

[corradio]

Note, the previous comment is also valid for UpdateView, when the error happens during template rendering, after context processing.