better system reply when calling "surfer" without having "surfer" installed

[seelisch]

Below you can see, what kind of reply the user gets right now. There should be a hint that most probably "surfer" is not installed, and that it can be downloaded for free at ​http://www.imaginary2008.de/data/surfer-setup.exe.

LIB "surf.lib";

loaded /usr/share/Singular/LIB/surf.lib (1.29,2008/12/12)

ring r=0,(x,y,z),lp; r;

characteristic : 0 number of vars : 3 block 1 : ordering lp : names x y z block 2 : ordering C

surfer(x2+y2+z2-4);

? surfer is not defined ? error occurred in STDIN line 4: surfer(x2+y2+z2-4);

[hannes]

No, your surf.lib ist not up-to-date( 1.31,2009/02/10 )

surfer(x2+y2+z2-4);

Close window to exit from 'surfer'

? calling surfer failed. (the shell return the error code 32512).

probably the executable surfer is not found.

? error occurred in surf.lib::surfer line 256: ERROR(err_mes); ? leaving surf.lib::surfer skipping text from ; error at token )