Attempting to delete an unlocked persistent volume does not tell the user anything useful

In tails-amd64-feature_14594-asp-gui-3.9-20180420T2055Z-e6f49b3f4f+devel@886427ce28.iso nightly ISO, “Delete Persistent Volume” is broken.

tails-persistence-delete.desktop[10550]: In fun get_variable_from_file: parameter 1 ($file): Reference bless( ["/var/lib/live/config/tails.persistence","/var/lib/l...) did not pass type constraint "Str" at /usr/share/perl5/Tails/Persistence/Setup.pm line 758.

Parent Task: #14595 (closed)

Original created by @alant on 15550 (Redmine)

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