Simplify a language string

I know that instantiate has a very specific meaning to developers but its pretty meaningless to regular folk. This PR simplifies the language string so that they might actually understand the error message if they are ever unlucky enough to see it


PHPMAILER_INSTANTIATE
Could not instantiate mail function.
==> Could not start ..

JLIB_UTIL_ERROR_APP_INSTANTIATION
Application Instantiation Error
==>Application Startup ...