add more logging

[kjk]

Debugging failures happening in a CI setup (like #3) is painful.
The more information about what's happening, the better.
Proposal: add vlog() (verbose log) function which prints to stdout if FUZZIT_VERBOSE env variable is set.
Log as much info as possible.

I can prepare a PR for that.

[kjk]

After doing a longer review of the code, all the error conditions are logged so there isn't much more to log that would be useful, so closing this issue.