Make sure directories exist about neonmodem

[mrusme]

@0-wiz-0 neonmodem is getting these directories at this point, using Go's os package. Those function use the environment variables set by the user and return those values. As written in the documentation:

UserCacheDir returns the default root directory to use for user-specific cached data. Users should create their own application-specific subdirectory within this one and use that.

On Unix systems, it returns $XDG_CACHE_HOME as specified by https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html if non-empty, else $HOME/.cache. On Darwin, it returns $HOME/Library/Caches. On Windows, it returns %LocalAppData%. On Plan 9, it returns $home/lib/cache.

If the location cannot be determined (for example, $HOME is not defined), then it will return an error.

I don't believe it is a good idea for a random program to just create the directories that a user has specified as ENV, e.g. $XDG_CACHE_HOME, hence the one thing that I found sensible was to let neonmodem crash loudly, so that the user takes care to fix his ENV.

However, having that said, I haven't read deeper into especially the XDG/Freedesktop specification to understand whether programs should in fact create these directories if they're non-existent.

If you have any info that explicitly points out that neonmodem is not behaving in a standard way free to re-open the issue. :-)