Graceful handling of parsing errors from template compiler

* The Play template compiler throws an exception as soon as a parsing error is
encountered. Call to the Play template compiler are now wrapped in a ``Try``
and its the responsability of the calling code to handle the failure case as it
makes sense.

* Use ``Annotation`` for reporting both parsing and compilation errors from
template files. The implication of this is that compilation  errors are not
persisted and are no longer displayed in the Problem View or the Package
Explorer.

* Because we now solely use Annotation, we no longer need the use the Eclipse
extension point for defining new markers (look at the deleted lines in
plugin.xml).

* Finally, I'm aware that the code could use some more clean-up, but my intention
was to minimize changes as we plan to release 0.1 possibly next week already.

Fixes #29