[engine][VELOCITY-917] Fix error message

git-svn-id: https://svn.apache.org/repos/asf/velocity/engine/branches/parser_experiments@1865656 13f79535-47bb-0310-9956-ffa450edef68

velocity-engine-core/src/main/java/org/apache/velocity/runtime/RuntimeInstance.java

diff --git a/velocity-engine-core/src/main/java/org/apache/velocity/runtime/RuntimeInstance.java b/velocity-engine-core/src/main/java/org/apache/velocity/runtime/RuntimeInstance.java
index 8b27fb7..ed5f919 100644
--- a/velocity-engine-core/src/main/java/org/apache/velocity/runtime/RuntimeInstance.java
+++ b/velocity-engine-core/src/main/java/org/apache/velocity/runtime/RuntimeInstance.java
@@ -407,7 +407,7 @@ private char getConfiguredCharacter(String configKey, char defaultChar)
         {
             if (configuredChar.length() != 1)
             {
-                throw new RuntimeException(configKey + " must be a single UTF-8 character");
+                throw new IllegalArgumentException(String.format("value of '%s' must be a single character string, but is '%s'", configKey, configuredChar));
             }
             return configuredChar.charAt(0);
         }