Latest master of WikiEditor with latest master of MediaWiki throws these warnings consistently.
It appears these keys are listed in the extension.json file of WikiEditor as part of the jquery.wikiEditor.toolbar.i18n module. But no such messages actually exist in the i18n system.
```
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-emdash' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-endash' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-minus' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-arabic' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-arabicextended' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-bangla' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-cyrillic' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-devanagari' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-greek' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-gujarati' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-hebrew' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-ipa' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-khmer' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-lao' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-latin' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-latinextended' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-persian' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-sinhala' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-symbols' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-tamil' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-telugu' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-characters-page-thai' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-help-content-heading1-description' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-help-content-heading1-result' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-help-content-heading1-syntax' (en)
MessageBlobStore::fetchMessage failed to find: 'wikieditor-toolbar-tool-file-pre' (en)
```