{{selection}}: Show an error message if nothing is selected.

[Taitava]

I tried to search for a discussion for this, but did not find. I thought I had written about this earlier, but maybe I don't remember correctly.

Currently, if {{selection}} is used while nothing is selected, this variable returns just an empty string. It might be ok for most situations, but I'd like to make {{selection}} able to utilize the default value system (started by #190 and will be improved by #298). If nothing is selected, user should be able to define their own selection value, e.g. No selection.

This change will mean that the default behavior of {{selection}} will change. By default, it will show an error message and prevent shell command execution, if nothing is selected. The error message should mention that this is new behavior for the variable, and that the old behavior (return an empty string and continue executing) can be restored by defining a global default value for {{selection}} in settings.

[Taitava]

This change was done in 0.18.0.