Report filename if File::open return Err

From issue #441

src/inputfile.rs

@@ -59,7 +59,10 @@ impl<'a> InputFile<'a> {
         match self {
             InputFile::StdIn => Ok(InputFileReader::new(stdin.lock())),
             InputFile::Ordinary(filename) => {
-                let file = File::open(filename)?;
+                let file = match File::open(filename) {
+                    Ok(f) => f,
+                    Err(e) => return Err(format!("{}: {}", filename, e).into()),
+                };
 
                 if file.metadata()?.is_dir() {
                     return Err(format!("'{}' is a directory.", filename).into());