Catch marshaller initialiation failure and report it more informatively.

src/java/com/samskivert/jdbc/depot/PersistenceContext.java

@@ -220,11 +220,16 @@ public class PersistenceContext
         throws PersistenceException
     {
         DepotMarshaller<T> marshaller = getRawMarshaller(type);
+        try {
             if (!marshaller.isInitialized()) {
-            // initialize the marshaller which may create or migrate the table for its underlying
+                // initialize the marshaller which may create or migrate the table for its
-            // persistent object
+                // underlying persistent object
                 marshaller.init(this);
             }
+        } catch (PersistenceException pe) {
+            throw (PersistenceException)new PersistenceException(
+                "Failed to initialize marshaller [type=" + type + "].").initCause(pe);
+        }
         return marshaller;
     }