If plantuml-default-exec-mode is 'server, the image is compiled via the
website and downloaded. If it's 'executable it uses the local 'plantuml'
executable (determined by plantuml-executable-path) to compile it. If
it's 'jar it uses the local 'plantuml.jar' (determined by
plantuml-jar-path).
The first two were tested and work, but 'jar did not, until now.
