Fix errcheck failures

Change-Id: Ic94b340796ee3ead1f1f9ab60726483ff741dd12

Author: skriptble

.errcheck-excludes

@@ -6,3 +6,4 @@
 (net.Conn).Close
 encoding/pem.Encode
 fmt.Fprintf
+fmt.Fprint

bson/extjson_builder.go

@@ -43,7 +43,10 @@ func ParseExtJSONArray(s string) (*Array, error) {
 	}
 
 	buf := make([]byte, b.RequiredBytes())
-	b.WriteDocument(buf)
+	_, err = b.WriteDocument(buf)
+	if err != nil {
+		return nil, err
+	}
 
 	doc, err := ReadDocument(buf)
 	if err != nil {