Do not swallow exceptions on failing to release a savepoint

By swallowing the exception on failing to release a savepoint, we are hiding
client code from database errors that they may need to be aware of or handle.

e.g. if the connection is now dead when trying to release the savepoint, by
swallowing the exception here, client code has no way of knowing this now until
the next operation on the database, which will yield a confusing message about
the problem and obscures where the failure occurred as it will be in a different
part of their code that throws any exception.

Swallowing the exception here is also inconsistent with all the other database
calls in the Transaction Object, where we always raise an exception up and let
client code decide how to handle the error.

spring-jdbc/src/main/java/org/springframework/jdbc/datasource/JdbcTransactionObjectSupport.java

@@ -186,7 +186,7 @@ public void releaseSavepoint(Object savepoint) throws TransactionException {
 			conHolder.getConnection().releaseSavepoint((Savepoint) savepoint);
 		}
 		catch (Throwable ex) {
-			logger.debug("Could not explicitly release JDBC savepoint", ex);
+			throw new TransactionSystemException("Could not explicitly release JDBC savepoint", ex);
 		}
 	}