Move XMLErrors.{cpp,h} from core/xml/ to core/xml/parser/.

Only XMLDocumentParser uses XMLErrors.

BUG=

Review URL: https://codereview.chromium.org/360673003

git-svn-id: svn://svn.chromium.org/blink/trunk@177203 bbb929c8-8fbe-4397-9dbb-9b2b20218538