Better error message if config (and/or HOME) directory is not writeable

[wischi-chr]

Is your feature request related to a problem? Please describe.
When gitui starts and there is no config and the home directory is write protected, it fails with an unspecific message: Error: Permission denied (os error 13) while trying to create folders and/or the config itself.

(Original Discussion for more context: #2682)

To Reproduce
(Simplest on Linux)

1. Spawn a new shell with a different home directory (just change HOME env variable)
   which is not writable (for example owned by root)
2. run gitui
3. Result: Error: Permission denied (os error 13)

Describe the solution you'd like
A clear and concise description of what gitui failed to do. So in this case probably something like:

Failed to write config to /some/path/.config
Error: Permission denied (os error 13)

Describe alternatives you've considered
Not implementing it and let the user figure it out :-D
I personally used strace to find the issue because I first thought it had something to do with my terminal setup (see #2682)

[sobczal2]

I would be happy to tackle this one if it is not yet taken