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;

lib/src/startup_helpers.rs

@@ -0,0 +1,20 @@
+pub fn report_permission_error() {
+    let user = std::env::var("USER").expect(
+        "Unexpected error reading value of $USER!");
+    let home = std::env::var("HOME").expect(
+        "Unexpected error reading value of $HOME!");
+    let current_executable = std::env::current_exe()
+        .expect("Unexpected error reporting executable path!");
+    eprintln!("USER: {}", user);
+    eprintln!("HOME: {}", home);
+    eprintln!("Executable: {}", current_executable.display());
+    if home == "/" {
+        eprintln!("User {} must have permission to write to '{}.zcash/' .",
+                  user,
+                  home);
+    } else {
+        eprintln!("User {} must have permission to write to '{}/.zcash/' .",
+                  user,
+                  home);
+    }
+}