message: Improve errors during `add_essential_files`
diff: @@ -10,7 +10,7 @@ use std::path::PathBuf;
  use std::fs::remove_dir_all;
  use postgres::Connection;
  use rustc_serialize::json::Json;
  -use error::Result;
  +use error::{Result, ResultExt};
  /// List of targets supported by docs.rs
  @@ -398,13 +398,15 @@ impl DocBuilder {
  let file_name = format!("{}-{}.{}", spl[0], rustc_version, spl[1]);
  let source_path = source.join(&file_name);
  let destination_path = destination.join(&file_name);
  - try!(copy(source_path, destination_path));
  + try!(copy(&source_path, &destination_path)
  + .chain_err(|| format!("couldn't copy '{}' to '{}'", source_path.display(), destination_path.display())));
  }
  for file in files.1.iter() {
  let source_path = source.join(file);
  let destination_path = destination.join(file);
  - try!(copy(source_path, destination_path));
  + try!(copy(&source_path, &destination_path)
  + .chain_err(|| format!("couldn't copy '{}' to '{}'", source_path.display(), destination_path.display())));
  }
  let conn = try!(connect_db());
  