Clean up ExceptionCode enumeration

Clean up ExceptionCode enumeration
https://bugs.webkit.org/show_bug.cgi?id=174812

Reviewed by Darin Adler.

Clean up ExceptionCode enumeration:
- Updated comments to reflect the latest WebIDL specification
- Drop special values given to some exception codes. Those legacy codes are already in the
  table inside DOMException.cpp. This also allows us to drop empty rows in the DOMException
  table
- Drop NoException value in the enumeration and use std::optional<ExceptionCode> when needed
  instead.

* Modules/indexeddb/shared/IDBError.cpp:
(WebCore::IDBError::IDBError):
(WebCore::IDBError::name):
(WebCore::IDBError::message):
* Modules/indexeddb/shared/IDBError.h:
(WebCore::IDBError::code):
(WebCore::IDBError::encode):
(WebCore::IDBError::decode):
* dom/DOMException.cpp:
* dom/ExceptionCode.h:
* fileapi/FileReaderSync.cpp:
(WebCore::errorCodeToException):
(WebCore::FileReaderSync::startLoading):
* xml/XMLHttpRequest.cpp:
(WebCore::XMLHttpRequest::createRequest):
* xml/XMLHttpRequest.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@219901 268f45cc-cd09-0410-ab3c-d52691b4dbfc