message: Use try operator when possible
diff: @@ -166,8 +166,7 @@ trait HeaderReader: BufRead {
  /// Reads the two magic constant bytes
  fn read_magic_constant(&mut self) -> ImageResult<[u8; 2]> {
  let mut magic: [u8; 2] = [0, 0];
  - self.read_exact(&mut magic)
  - .map_err(ImageError::IoError)?;
  + self.read_exact(&mut magic)?;
  Ok(magic)
  }
  @@ -226,8 +225,7 @@ trait HeaderReader: BufRead {
  /// Read the next line
  fn read_next_line(&mut self) -> ImageResult<String> {
  let mut buffer = String::new();
  - self.read_line(&mut buffer)
  - .map_err(ImageError::IoError)?;
  + self.read_line(&mut buffer)?;
  Ok(buffer)
  }
  @@ -300,7 +298,7 @@ trait HeaderReader: BufRead {
  let mut tupltype: Option<String> = None;
  loop {
  line.truncate(0);
  - let len = self.read_line(&mut line).map_err(ImageError::IoError)?;
  + let len = self.read_line(&mut line)?;
  if len == 0 {
  return Err(ImageError::Decoding(DecodingError::with_message(
  ImageFormat::Pnm.into(),
  