fix: prevent segfault when template does not exist

[Piglit]

setTemplate now produces an error, if the chosen template does not exist. But the game no longer crashes with a segfault.

[daid]

I'm quite sure this moves segfaults to other spots, there are quite some places in the code that assume that the template property is set.

[Piglit]

Actually it produces quite nice errors and defined behaviour, since objects without templates get deleted in ShipTemplateBasedObject::update:

ERROR: Failed to find ship template: AtlantisXX
ERROR: Failed to find ship template: Adder MK5XX
ERROR: Failed to find ship template: 
ERROR: ShipTemplateBasedObject with ID 241 lacked a template, so it was destroyed.
ERROR: Failed to find ship template: 
ERROR: ShipTemplateBasedObject with ID 246 lacked a template, so it was destroyed.

[daid]

In that case, it's mostly fine. There is still a chance that it runs into a crash because other code doing something with the object before the update runs, but those are then extreme edge cases