Add context to i/o errors
diff --git a/gen/build/src/lib.rs b/gen/build/src/lib.rs
index 840424e..8f48f20 100644
--- a/gen/build/src/lib.rs
+++ b/gen/build/src/lib.rs
@@ -59,8 +59,7 @@
 
 use crate::error::Result;
 use crate::gen::error::report;
-use crate::gen::Opt;
-use std::fs;
+use crate::gen::{fs, Opt};
 use std::io::{self, Write};
 use std::iter;
 use std::path::Path;