Added check that template mode actually exists
Closes #177. Removed leniency flag.
All dialects are now considered lenient.
The "isLenient" method has been removed from the IDialect interface, but a default implementation
(returning always "true") has been added to AbstractDialect for backwards compatibility, in order
to avoid migration problems.
Anyway, even if dialects return "false" for this method, they will always be considered lenient.