Refers to #18

Closes #28

Refers to #18, #28

Refers to #18, #28

Refers to #28

This way we can handle the situation in the calling code, e.g. sending
an email notification to the admin address.

Locating the update package is now done in Find-InstallationPackage and
stored in $UpdPackage, so we can safely remove the redundant code. In
addition, this change makes the function return $True or $False,
depending if the copy action succeeded.

Refers to #28

Refers to #18, #28

After validating the config files we can now safely trigger the update
if necessary.

Refers to #18, #28

No checks are done in Update-Configuration any more, as the control
logic has been changed to do those beforehand now. Therefore also
Update-File is not comparing the write-times any more nor terminating
the script if something fails (now returning $False instead).

Refers to #18, #28

Instead of terminating the entire script immediately we log a message
and pass on success / fail so the remaining script logic can deal with
the situation.

Refers to #18, #28

This adapts the updater to the new configuration locations (#18) and
changes the control flow so that first the script will detect if any
updates are necessary at all and then validate the configuration files
(respecting what parts will be updated), preparing for the actual update
tasks (as this is currently work-in-progress, those update tasks are
disabled for now.

Refers to #28

The updater should also check if an existing configuration that won't be
updated validates correctly with newer service binaries and stop the
process of updating if the check fails.

Refers to #28

Refers to #28

Refers to #18

Implements the core functionality of #28

This is required to correctly propagate exceptions from function calls.
Otherwise every call has to specify the "-EA" flag explicitly.

Refers to #28

Refers to #28, #13

Refers to #28, #18, #13

Refers to #17

Closes #27

Refers to #13, #17

Refers to #3, #17

Refers to #13

Refers to #13

And only start the service eventually if it was running before.

Only check if the base folder exists.

With this commit the updater script is considered as being completed for
now, so this fixes #13.