20: Handle missing variables gracefully

src/main/tjava/com/r35157/jupiterperpsalarm/impl/ref/PriceAlarm.tjava

@@ -22,9 +22,19 @@ public final class PriceAlarm {
             );
         }
 
-        BigDecimal target = AlarmTargetParser.parse(
+        BigDecimal target;
+        try {
+            target = AlarmTargetParser.parse(
                     variableResolver.resolve(definition.targetExpression())
             );
+        } catch (RuntimeException exception) {
+            System.err.printf(
+                    "Could not resolve target for alarm %d: %s%n",
+                    definition.id(),
+                    exception.getMessage()
+            );
+            return;
+        }
 
         boolean reached = definition.direction().reached(
                 price.priceUsd(),