Help Wanted: Use `github.com/pkg/errors` for all server error handling

[matterdoc]

Mattermost user ali.farooq from https://community-daily.mattermost.com has requested the following be documented:

Maybe this decision is worth documenting in https://developers.mattermost.com/contribute/server/, as a lot of people are confused by what they should use.

See the original post here.

Since this is a stylistic request, might be worth documenting under https://docs.mattermost.com/developer/style-guide.html

This issue was generated from Mattermost using the Doc Up plugin.

[cwarnermm]

@hanzei - This Help Wanted ticket has been idle for some time. Would you be interested in creating a PR with the requested documentation updates?

[hanzei]

@cwarnermm I'm happy to keep this as a HW.

[cwarnermm]

Sounds great, @hanzei. Thanks!