Improve error message when mjml is not installed

[nicolas-brousse]

In a server I've got an issue where mails was not sent anymore.

We've got the following error, because mjml was not installed (it was before):

ActionView::Template::Error
No such file or directory - 

I think #extract_executable_version method needs to catch Errno::ENOENT exception, or something like that.

It may help a lot for debug.

In our case, on each deploys packages are installed. But some server changes made that yarn install doesn't installed mjml.
For me the main issue here was that the error message came from ActionMailer and not MJML gem.

[m-basov]

@nicolas-brousse thank you for pointing this out! I do not have time to work on this project yet so if you can make PR with better error it may help someone in the future! :)

[nicolas-brousse]

@kolybasov I'll try to have a look and open a PR this week if I found a bit of time