Replace fmt.Errorf with errors.Errorf

[martinmr]

Fixes #3449

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This change is 

[campoy]

Reviewed 5 of 50 files at r1.
Reviewable status: 5 of 50 files reviewed, 3 unresolved discussions (waiting on @campoy, @danielmai, and @martinmr)

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conn/pool.go, line 39 at r2 (raw file):

	ErrNoConnection = errors.Errorf("No connection exists")
	// ErrUnhealthyConnection indicates the connection to a node is unhealthy.
	ErrUnhealthyConnection = errors.Errorf("Unhealthy connection")

These should be errors.New technically ... but I don't think it matters much, tbh

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dgraph/cmd/alpha/http.go, line 108 at r2 (raw file):

	uintVal, err := strconv.ParseUint(value, 0, 64)
	if err != nil {
		return 0, errors.Errorf("Error: %+v while parsing %s as uint64", err, name)

These should be replaced with errors.Wrapf(err, "could not parse %q as unit64", name)

And same for all the other fmt.Errorf which embed an error.

[martinmr]

Reviewable status: 4 of 50 files reviewed, 3 unresolved discussions (waiting on @campoy, @danielmai, and @golangcibot)

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conn/pool.go, line 39 at r2 (raw file):

Previously, campoy (Francesc Campoy) wrote…

These should be errors.New technically ... but I don't think it matters much, tbh

Just changed this since they are typed errors. I left the rest as-is.

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dgraph/cmd/alpha/http.go, line 108 at r2 (raw file):

Previously, campoy (Francesc Campoy) wrote…

These should be replaced with errors.Wrapf(err, "could not parse %q as unit64", name)

And same for all the other fmt.Errorf which embed an error.

Done.

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dgraph/cmd/migrate/table_guide.go, line 24 at r1 (raw file):

Previously, golangcibot (Bot from GolangCI) wrote…

File is not gofmt-ed with -s (from gofmt)

Done.

addressed comments

[campoy]

Reviewed 44 of 50 files at r1, 1 of 1 files at r2, 22 of 22 files at r3.
Reviewable status: all files reviewed, 3 unresolved discussions (waiting on @campoy, @danielmai, and @golangcibot)