Clarify usage with regard to "root folder"

[martinlindhe]

The "root folder" setting should be the same folder where the built MacroQuest.exe resides.

For example, with a checkout in D:\dev-mq\macroquest-rof2, then the "root folder" should be

"D:\dev-mq\macroquest-rof2\build\bin\release".

When I incorrectly entered the root folder of the git repository (D:\dev-mq\macroquest-rof2), then I could generate navmesh, and I could save navmesh. No errors are shown, no errors in the "log window", and no file is written to disk.

For improved user experience, it would be helpful to either (1 or more of):

1. Improve documentation about what is the root folder, perhaps improve the wording here:

2. Show something like "Saving navmesh to " in the log window.

3. Show an error if navmesh failed to be written to disk.

I have verified that the .navmesh was not written elsewhere in my source tree, like this:

$ fd -u -e navmesh
build/bin/release/resources/MQ2Nav/wallofslaughter.navmesh