Document / add example using github.com/rotisserie/eris

[rhcarvalho]

https://github.com/rotisserie/eris is an alternative to https://github.com/pkg/errors and https://github.com/golang/xerrors, should work out of the box with Sentry.

They have an example app to report errors with Sentry.

[ribice]

This works out of the box. eris errors implement both Unwrap and Cause methods.

We check whether an error implements Unwrap, and if so get the unwrapped one.

cc @cleptric

[cleptric]

Worth adding to https://docs.sentry.io/platforms/go/usage/#capturing-errors.

[ribice]

Worth adding to https://docs.sentry.io/platforms/go/usage/#capturing-errors.

getsentry/sentry-docs#12199