Make dgraphloader pass errors up

[srh]

We were aborting instead of expressing useful output semi-randomly when the user pressed Ctrl+C.

Connected to #1376.

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This change is 

[srh]

Review status: 0 of 1 files reviewed at latest revision, 2 unresolved discussions.

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cmd/dgraphloader/main.go, line 167 at r1 (raw file):

		err = readLine(bufReader, &buf)
		if err != nil {
			if err != io.EOF {

Moved this up here (without changing behavior of the code, other than that the value gets returned) because it's more readable (to see that this is the only err case that flows into the != EOF check).

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cmd/dgraphloader/main.go, line 199 at r1 (raw file):

		if batchSize >= *numRdf {
			if err = dgraphClient.BatchSetWithMark(r, absPath, line); err != nil {
				return err

Sacrificed some error messaging for simplicity, since we have err != context.Canceled logic elsewhere.

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Comments from Reviewable

[pawanrawal]

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Review status: 0 of 1 files reviewed at latest revision, 2 unresolved discussions.

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Comments from Reviewable

[srh]

In master as of 34cd170.