better explanation how to configure external editor (#382)

[akshay1992kalbhor]

Any suggestions will be appreciated. This is kind of a first draft.

[WizardOhio24]

Thanks for the PR.
With this, should it not show the user the actual error because it might not be that GIT_EDITOR is not set, it could be something entirely different, I mean, in the original issue it said vi could not be found, but that could be for a number of reasons, not just GIT_EDITOR.
I'd say as well, it should include "and the editor is installed and valid.", if the user doesn't have micro installed, they can't use micro, no matter if GIT_EDITOR is set.

So, the error message then tells the user the most common way to fix it, but also shows the user the error.

What do you think?

[extrawurst]

@akshay1992kalbhor thanks for your interest in supporting gitui.
can you check the new behaviour for when some editor env variable is empty: 156776a

for the time being I mark this as draft

[extrawurst]

seems unmaintained, feel free to update and re-open