Enable the errcheck linter

[timvaillancourt]

Description

This PR enables the errcheck linter in the project golangci-lint config:

errcheck is a program for checking for unchecked errors in go programs.

Also a lot of missing error handling was added for problems (or // nolint:errcheck flags for some false-positives). For the most part I have avoided introducing new scenarios where we fail/exit the migration, so in many cases I just-added a log.Errorf(...) without modifying the return

In case this PR introduced Go code changes:

• contributed code is using same conventions as original code
• script/cibuild returns with no formatting errors, build errors or unit test errors.

[dm-2]

We're slowly replicating the entire ticker loop that follows, perhaps we should refactor initiateThrottlerCollection to something along the lines of the below - what do you think?

(notifyFirstThrottlingCollected could probably be better named!)

func (this *Throttler) initiateThrottlerCollection(notifyFirstThrottlingCollected chan<- bool) {
	go this.collectReplicationLag(notifyFirstThrottlingCollected)
	go this.collectControlReplicasLag()
	go this.collectThrottleHTTPStatus(notifyFirstThrottlingCollected)
        firstMetricsCollected := false

	go func() {
		ticker := time.NewTicker(time.Second)
		defer ticker.Stop()
		for range ticker.C {
			if atomic.LoadInt64(&this.finishedMigrating) > 0 {
				return
			}

			err := this.collectGeneralThrottleMetrics()
			if err != nil {
				this.migrationContext.Log.Errorf("Failed to collect throttler metrics: %+v", err)
			} else if !firstMetricsCollected {
				notifyFirstThrottlingCollected <- true
				firstMetricsCollected = true
			}
		}
	}()
}

[dm-2]

It looks like we never actually use the string return value from the WriteAndLogChangelog, WriteChangelogState, and WriteChangelog functions - what do you think about getting rid of it and just returning error?