As a Wikidata editor, I want the UI to show me useful and comprehensible error messages when I do something wrong.

Problem:
With the geoshape or tabular data datatypes, if you try to save a value for a data page that doesn’t exist on Commons, you get an error. That error message is broken:

Could not save due to an error.

⧼wikibase-validator-page-not-exist⧽

Example:

Screenshots/mockups:

BDD
GIVEN a Data: filename that does not exist on Commons
WHEN I try to add it on some item as a statement for a geoshape or tabular data property
THEN I get a human-readable error message

Acceptance criteria:

• The error message works.

Notes:

• The correct error message is “page-not-exists” (wikibase-validator-page-not-exists in repo/i18n/en.json and translations). InterWikiLinkExistsValidator has “page-not-exist”.