refactor: use recently introduced StringSubstitutor.setEnableUndefinedVariableException()

Follow-up to #1182

Author: php-coder

src/main/java/ru/mystamps/web/feature/report/ReportServiceImpl.java

@@ -73,7 +73,9 @@ public String prepareDailyStatistics(AdminDailyReport report) {
 		put(ctx, "invalid_csrf_cnt", report.getInvalidCsrfCounter());
 		put(ctx, "bad_request_cnt", -1L);  // FIXME: #122
 
-		return new StringSubstitutor(ctx).replace(template);
+		StringSubstitutor substitutor = new StringSubstitutor(ctx);
+		substitutor.setEnableUndefinedVariableException(true);
+		return substitutor.replace(template);
 	}
 
 	private static void put(Map<String, String> ctx, String key, long value) {

src/main/java/ru/mystamps/web/feature/site/MailServiceImpl.java

@@ -135,6 +135,7 @@ private String getTextOfActivationMail(SendUsersActivationDto activation) {
 		ctx.put("expire_after_days", String.valueOf(CronService.PURGE_AFTER_DAYS));
 
 		StringSubstitutor substitutor = new StringSubstitutor(ctx);
+		substitutor.setEnableUndefinedVariableException(true);
 		return substitutor.replace(template);
 	}
 
@@ -151,6 +152,7 @@ private String getSubjectOfDailyStatisticsMail(AdminDailyReport report) {
 		ctx.put("total_changes", String.valueOf(report.countTotalChanges()));
 
 		StringSubstitutor substitutor = new StringSubstitutor(ctx);
+		substitutor.setEnableUndefinedVariableException(true);
 		return substitutor.replace(template);
 	}