Do not remove unused imports if unit has prior errors.

In this situation JDT will return an incomplete AST and
UnusedImportsRemover will (incorrectly) remove referred imports,
triggering the error reporting logic to report errors of the form
"<name> can not be resolved to a type" for the types whose imports
were removed.

Change-Id: I42642237b0df7e56b26041a729eff921d8bd2982
3 files changed