Misleading OSError message when saving an animation to an inexistent folder

[seblabbe]

From this ask question, the command

sines = [plot(c*sin(x), (-2*pi,2*pi), color=Color(c,0,0), ymin=-1, ymax=1)
for c in srange(0,1,.1)]
a = animate(sines)
a.save('animations/sines')

returns

Command "['convert', '-dispose', 'Background', '-delay', '20', '-loop',
'0', '*.png', '/home/slabbe/animations/sines.gif']" returned non-zero 
exit status "1" (with stderr "convert-im6.q16: unable to open image 
`/home/slabbe/animations/sines.gif': Aucun fichier ou dossier de ce 
type @ error/blob.c/OpenBlob/2874." and stdout "").
---------------------------------------------------------------------------
Traceback (most recent call last):
...
OSError: Error: Cannot generate GIF animation. The convert command
(ImageMagick) is present but does not seem to be functional. Verify 
that the objects passed to the animate command can be saved in PNG 
image format. See www.imagemagick.org more information.

The problem is not that imagemagick is not functional.

The way error message are given as written in #33092 needs to be improved.

CC: @sagetrac-tmonteil

Component: graphics

Author: Sébastien Labbé

Branch/Commit: u/slabbe/34204 @ bc5851c

Issue created by migration from https://trac.sagemath.org/ticket/34204

[seblabbe]

Commit: bc5851c

[seblabbe]

Branch: u/slabbe/34204

[seblabbe]

New commits:

02b5205	34204: better error message when pathhead of savefile does not exist
bc5851c	34204: moving the require() command just before features ffmpeg and imagemagick are used

[seblabbe]

Author: Sébastien Labbé

[seblabbe]

comment:2

Both:

sage -t src/sage/plot/animate.py
sage -t --optional=sage,optional,external src/sage/plot/animate.py

works for me. My setup is that I have both ffmpeg and imagemagick installed.

However, newly added doctests do not depend on ffmpeg or imagemagick because these features are "required" only after the new error message is raised.

Needs review.