Make wrong file size fatal error optional

If the file is short, this change creates a warning instead of an error. The -s option causes it to fail instead of warn. If desired, you could easily reverse the sense of -s to make it nonfatal, although my preference is to make it a warning since I don't always fill a PIC or Atmel device.

Also removed the redundant short read messages and reduced repeated calls to getfilesize in the size checks.

Thanks!