Bulk multi-shard DBs fix

[codexnull]

Change summary:

• Each shard's schema contains only the predicates found in that shard rather than all preds.

• Test case for above fix.

• More user-friendly "file not found" messages rather than stack traces when bulk schema or data paths invalid.

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This change is 

[martinmr]

Reviewed 4 of 4 files at r2.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @codexnull)

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dgraph/cmd/bulk/loader.go, line 246 at r2 (raw file):

m 

rename this variable to something a bit more descriptive. m implies a map but we are using this more as a set.

Ideas: usedPreds, fullPreds, seenPreds or something like that.

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dgraph/cmd/bulk/run.go, line 136 at r2 (raw file):

		os.Exit(1)
	} else if _, err := os.Stat(opt.DataFiles); err != nil && os.IsNotExist(err) {
		fmt.Fprintf(os.Stderr, "Data path(%v) does not exist .\n", opt.DataFiles)

Remove the blank space between exist and the period.

[manishrjain]

Merge after addressing @martinmr 's comments.

Reviewed 2 of 4 files at r2, 2 of 2 files at r3.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @codexnull)

[codexnull]

Reviewable status: all files reviewed, 2 unresolved discussions (waiting on @martinmr)

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dgraph/cmd/bulk/loader.go, line 246 at r2 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

m 

rename this variable to something a bit more descriptive. m implies a map but we are using this more as a set.

Ideas: usedPreds, fullPreds, seenPreds or something like that.

I used m because it matches ld.schema.m below.

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dgraph/cmd/bulk/run.go, line 136 at r2 (raw file):

Previously, martinmr (Martin Martinez Rivera) wrote…

Remove the blank space between exist and the period.

Done.