Add ability for gnuplot to display/send back errors

[abchk1234]

Hi, While trying out GnuPlot, I faced following issue:

• gnuplot was not correctly installed. However there was no error thrown. I looked in logs, even enabled display_errors, yet no output.
• Finally I got gnuplot working and using php write.php from command line worked, but running it via web failed. Again, I got no output from script to let me know there was an error somewhere.

Could errors from running gnuplot be passed back to the user, please?