Proposal: Add static checker to ensure return when an error occurs in api handlers

[AGMETEOR]

Summary

This PR adds code for a new static checker that checks whether when an error occurs in api handler, we return.

See some cases below:

mattermost-vet will flag this

func a(c *Context, w http.ResponseWriter, r *http.Request) {
	err := iR()
	if err != nil { // not ok
		c.Err = err
	}
}

mattermost-vet will not flag this

func b(c *Context, w http.ResponseWriter, r *http.Request) {
	err := iR()
	if err != nil { // ok
		c.Err = err
                 return
	}
}

mattermost-vet will not include this static checker in this handler

// skip appErrReturn check
func q(c *Context, w http.ResponseWriter, r *http.Request) {
	err := iR()
	t := iR()
	s := true

	if t == nil && (err != nil) && s { // not ok but skipped
		c.Err = err
	}
}

[agnivade]

Finally some updates :) Let me know whenever it is ready for review. Would be happy to take a look.

[AGMETEOR]

Finally some updates :) Let me know whenever it is ready for review. Would be happy to take a look.

Hey @agnivade , sure. Will let you know. Hopefully by Monday will be done and have a fully working analyzer

[mattermost-build]

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