optparse 1.7.3

• The errors raised by parse_args() (and parse_args2()) are now of class "optparse_parse_error".

  When interactive() is FALSE we now print out a usage string followed by
  a (less verbose) error message.

• Throws a more informative error message for unknown short flags when positional_arguments=TRUE.
  Thanks Greg Minshall for bug report (#42).