OptparsePlus Internal - treat as private

For RBX; it throws a different exception when a command isn't found, so we override that here.

Methods
E
Instance Public methods
exception_meaning_command_not_found()

Source: show 

# File lib/optparse_plus/execution_strategy/rbx_open_4.rb, line 7
def exception_meaning_command_not_found
  [Errno::EINVAL] + Array(super)
end