Improve exception handling in SpecialModifyEntity

SpecialPageException does not exist. Also, entity id
parsing can result in either a ParseException or
EntityIdParsingException (both derived from RuntimeException).

Change-Id: I3f7ab7e1b3c8a8aed0265915382f9a9eda84e2fe
Merge "Improve exception handling in SpecialModifyEntity"