make startup file creation fail more explicitly (#7)

* add explicit permission error handling

* create error_helpers as mod to keep main.rs succint

* rename error_helpers -> startup_helpers

lib/src/lib.rs

@@ -1,6 +1,7 @@
 #[macro_use]
 extern crate rust_embed;
 
+pub mod startup_helpers;
 pub mod lightclient;
 pub mod grpcconnector;
 pub mod lightwallet;