If an exception is hit in the init method, DocumentsWriter incorrectly
increments numDocsInRAM when in fact the document is not added.
Spinoff of this thread:
The root cause that led to the exception in init was actually due to
incorrect use of Lucene's APIs (one thread still modifying the
Document while IndexWriter.addDocument is adding it) but still we
should protect against any exceptions coming out of init.