Use error returns and interface type for parsing

Fixes issue #72

This change simplifies interactions with parsing N-Quads and makes
reading datasets more robust. Changes made while here also improve
performance:

benchmark           old ns/op     new ns/op     delta
BenchmarkParser     1058          667           -36.96%

We still use string concatenation which I'm not wildly happy about, but
I think this can be left for a later change.

Initial changes towards idiomatic error handling have been made. More
significant changes are needed, but these have subtle design implication
and need to be thought about more.

30kmoviesdata.nt.gz has been altered to properly escape double quotes.
This was done mechanically and with manual curation to pick up
straglers.

nquads/nquads_test.go

@@ -25,17 +25,26 @@ var testNTriples = []struct {
 	message string
 	input   string
 	expect  *graph.Triple
+	err     error
 }{
 	// NTriple tests.
 	{
 		message: "not parse invalid triples",
 		input:   "invalid",
 		expect:  nil,
+		err:     ErrAbsentPredicate,
+	},
+	{
+		message: "invalid internal quote",
+		input:   `":103032" "/film/performance/character" "Walter "Teacher" Cole" .`,
+		expect:  nil,
+		err:     ErrUnterminated,
 	},
 	{
 		message: "not parse comments",
 		input:   "# nominally valid triple .",
 		expect:  nil,
+		err:     nil,
 	},
 	{
 		message: "parse simple triples",
@@ -110,7 +119,10 @@ var testNTriples = []struct {
 
 func TestParse(t *testing.T) {
 	for _, test := range testNTriples {
-		got := Parse(test.input)
+		got, err := Parse(test.input)
+		if err != test.err {
+			t.Errorf("Unexpected error when %s: got:%v expect:%v", test.message, err, test.err)
+		}
 		if !reflect.DeepEqual(got, test.expect) {
 			t.Errorf("Failed to %s, %q, got:%q expect:%q", test.message, test.input, got, test.expect)
 		}
@@ -121,6 +133,6 @@ var result *graph.Triple
 
 func BenchmarkParser(b *testing.B) {
 	for n := 0; n < b.N; n++ {
-		result = Parse("<http://example/s> <http://example/p> \"object of some real\\tlength\"@en . # comment")
+		result, _ = Parse("<http://example/s> <http://example/p> \"object of some real\\tlength\"@en . # comment")
 	}
 }