Fix lint issues

.gometalinter.json

+{
+    "aggregate": true,
+    "vendor": true
+}

Gopkg.lock

 [solve-meta]
   analyzer-name = "dep"
   analyzer-version = 1
-  inputs-digest = "36bcaf7701d70b8c4648e01a7fc94e0cda9c2f9b48c42d453c87567f6f058036"
+  inputs-digest = "24afdd6b64331aeba47fed75918d04032e13e404612cac107bad1d68a5038b72"
   solver-name = "gps-cdcl"
   solver-version = 1

main.go

 }
 
 func usage(msg string) {
-	io.WriteString(os.Stderr, msg+"\n")
+	io.WriteString(os.Stderr, msg+"\n") // nolint: errcheck
 	os.Exit(1)
 }