GROOVY-3457: Fixing javadocs and unit tests for StreamingTemplateEngine

A follow up on pull requests [503](https://github.com/groovy/groovy-core/pull/503) and [533](https://github.com/groovy/groovy-core/pull/533). 

Fixed a few bugs in error reporting when template parse/execution errors are encountered, added unit test to test the error reporting, finalized javadocs for StreamingTemplateEngine. 

*Question*: Should we rename the template engine to something more to the point. There is no 'streaming' going on here. We just build closures under the hood. Streaming was the best I could come up with at the time but I am open to suggestions.