more sensible error when adding to a non-existing observable

src/measurements.h

@@ -32,6 +32,9 @@ private:
 
 template<class T>
 void measurements::add(const std::string name, T value) {
+	if(observables_.count(name) == 0) {
+		throw std::runtime_error{fmt::format("tried to add to observable '{}' which was not registered!", name)};
+	}
 	observables_.at(name).add(value);
 }
 }