showterm.el: use new magick command by default, fallback to convert

[drzv]

A better fix for #3337

I used "convert" as a string in the fallback case instead of (executable-find "convert") because it provides a more useful error message in case neither command is found.

[triska]

Thank you a lot!

I think this makes it worth considering to also revert #3338, because make-process requires Emacs version 25 or later, whereas start-process works also in much older Emacs versions.

[drzv]

Good point, I reverted that commit.

[triska]

Thank you, almost perfect! Two small issues remain: First, this does not need let*, it suffices to use let. But even that is not necessary, we could write this for example as:

(start-process "convert" (current-buffer)
               (or (executable-find "magick") "convert"))
                            ...

[drzv]

I think it did need let*, but your solution is better either way. Thank you.

[triska]

Ah yes, of course, completely right! Awesome, thank you a lot!