scripts: refactoring & miscellaneous small changes (#2499)

* `update-check.sh` now uses the new log

* refactored `setup-stack.sh`

The changes are:

1. Replaced `""` wiht `''` where possible (reasoning: Bash is very
   implicit and I'd like to use `''` where possible to indicate no
   variables are expanded here)
2. `> /file` -> `>/file` according to our style guide
3. Some log adjustments for messages where I deemed it appropriate
4. Then, an error message from a Dovecot setup was also prevented (by
   adding a check whether the directory is present before a `: >...`
   command would create a file in this directory).

These are all small, miscellaneous changes that I wanted to combine into
one commit and ultimately one PR because I see no point in opening a PR
for every small change here. I hope this is fine.

* added a small `sleep` to the `_shutdown` function

This ensure the last log message is actually logged before Supervisor
logs the message that it received a SIGTERM. This makes reading the log
easier because now the causal relationship is shown (we are terminating
Supervisor, and not someone else and we're just logging it).

I forgot to replace `""` with `''` in `update-check.sh`, so I included
it here because this is the last commit before PR review.

* re-add exit on successful update (only)

* re-added date information to update-check log messages

* added `_log_with_date` function

The new function will log a message with a proper timestamp. This is all
handled in `log.sh`, we therefore not need to source other files too.

This will be used in the future by `check-for-changes.sh` as well :)

Co-authored-by: Casper <casperklein@users.noreply.github.com>
Co-authored-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

target/scripts/helpers/error.sh

@@ -76,6 +76,7 @@ function _shutdown
   _log 'error' "${1}"
   _log 'error' 'Shutting down'
 
+  sleep 1
   kill 1
   exit 1
 }