Add test for extracting a Java AST with an error expression

Also note that ErrorExpr can occur outside upgrade/downgrade scripts

java/ql/lib/semmle/code/java/Expr.qll

@@ -276,7 +276,8 @@ class ExprParent extends @exprparent, Top { }
  * An error expression.
  *
  * These may be generated by upgrade or downgrade scripts when databases
- * cannot be fully converted.
+ * cannot be fully converted, or generated by the extractor when extracting
+ * source code containing errors.
  */
 class ErrorExpr extends Expr, @errorexpr {
   override string toString() { result = "<error expr>" }