adjust errors and logger (#10)

internal/errors/http_errors.go

@@ -0,0 +1,11 @@
+package errors
+
+import "net/http"
+
+func InvalidArg(arg string) error {
+	return Newf(nil, http.StatusBadRequest, "invalid argument: %s", arg)
+}
+
+func HTTPShutDown(cause error) error {
+	return Newf(cause, http.StatusInternalServerError, "http server shut down")
+}