boostrap: dist: if a file cannot be installed because it does not exist, print its name in the error message.

src/bootstrap/lib.rs

@@ -1288,6 +1288,9 @@ impl Build {
         t!(fs::create_dir_all(dstdir));
         drop(fs::remove_file(&dst));
         {
+            if !src.exists() {
+                panic!("Error: File \"{}\" not found!", src.display());
+            }
             let mut s = t!(fs::File::open(&src));
             let mut d = t!(fs::File::create(&dst));
             io::copy(&mut s, &mut d).expect("failed to copy");