displaying an error if gnuplot is not found

[John W. Eaton]

On 16-Apr-2008, Søren Hauberg wrote:

| I think you want a newline (\n) at the end of the error message.
| Otherwise the error appears at the top of a very long list of errors. If
| the point is to help people who don't read the error messages, then I
| think we need the newline.

OK.

jwe