non-interactive usage and color error messages

[Petr M. <mi...@ph...>]

It is neither nice or useful when gnuplot is writing error message such as:

Error: terminal "dumb" does not support continuous colors.
This terminal does not support palette-based images.
This terminal does not support rgb images.
This terminal does not support filled polygons.

when in non-interactive mode (e.g. driven via Octave). They are actually 
just warnings, not errors.

I propose to surround the respective code by

if (interactive)
  ...error message...

What do you think? Are there any other messages that would deserve the same 
care?

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PM