[xtext][formatter] Incorporated review result

- Moved nested types to top level types
- Moved API exception from *internal to API package
- Moved ToString helpers to *debug package
- Renamed ExceptionAcceptor#NULL to #IGNORING
- Minor JavaDoc addition

Change-Id: If204bbbb4bab8e775c6ecb642afce60cf3c8699c
Merge "[xtext][formatter] Incorporated review result"