Improving error messages: When Elm is not found

[jwoLondon]

If there has been a problem installing Elm as part of Litvis, trying to preview a litvis document that contains some elm generates this error message:

spawn elm ENOENT (litvis:elmp-environment)

We should replace this with a more friendly message, especially as this is most likely to occur when using Litvis for the first time. I suggest

I am having trouble finding Elm on your machine. Is it installed? Check by opening a terminal window and typing "elm --version" (without quotation marks). If you have recently installed Elm, try restarting your machine.

[kachkaev]

This makes sense and is doable. The only mini-challenge I'm in the middle of solving is to make ENOENT errors non-cachable (this would be an exceptional case).

[kachkaev]

As it turns out, I’ve already had a mechanism of expiring working directories with temporary elm projects. This issue should be fixed now on master and will be available to users with the next release.