Graceful handling of parsing errors from template compiler

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).

Fixes #29