Change error message on destroying active pclaims

[Granger]

Simple thing: the error message when executing destroy on an active stake claim implies that it can't be destroyed.

In case there a means to get rid of an active stake claim the system message should hint them.

[Kamekono]

But is there a mean to destroy one?

[loftar]

I said elsewhere that it was possible to destroy claim stakes with siege engines, but apparently I was talking out of my rear side. It quite arguably should be, though.