Give better error message for editor enviroment variable if variable is empty

[WizardOhio24]

This is link to a discussion in #419.

This gives a better error message for opening an external editor. This is also linked to #400, but is different because this gives a specific error message, saying the one of the environment variables must be empty.

[pm100]

here is what git says

[WizardOhio24]

I do not get that. Look at the lines changed, this PR could not cause that, it only changes a string which would be output in the event of an error, where the error can only be if the environment variables are empty:

To see this, use the command GIT_EDITOR="" cargo run and try to open the editor(for example, by committing then ^e). What did you do to produce that error? It is definitely unrelated to this considering what this changes, so there should be another PR for it.

[WizardOhio24]

Wait, ignore my last comment, just realised you meant git cli.

That's fine for git to say that, but it is a cli and GitUI is a UI and so should try to be eaiser to use than git, that's the whole point. And the error that git gives is not helpful if an environment variable is not set correctly. This PR gives the user a better error message so they should be able to resolve the issue without ever having to go to the internet. I imagine that if a git cli user saw that git cli error and was relatively unfamiliar with git, they would spend a bit of time searching until they found the answer.

[extrawurst]

@WizardOhio24 sorry that my review delay seem to offend you. closing without any comments seems not constructive. lets keep this open until its reviewed

[extrawurst]

closed in favour of 156776a