An array of valid value types is not respected in error message

[FlorianSW]

If an allowed value type for a property is defined as an array, like:

"ExtensionFunctions": {
    "type": [
        "array",
        "string"
    ],
    "description": "Function to call after setup has finished",
    "items": {
        "type": "string"
    }
},

the error message, if a value does not conform to this schema, is somtheing like:
[ExtensionFunctions] Object value found, but a string is required

while it should be something like:
[ExtensionFunctions] Object value found, but an array, a string or an integer is required

The error message is misleading, as it indicates, that only the type string is valid, while in fact other types are valid, too. The error mesage should indicate all valid types, instead of just one of them.

Original report in Wikimedia Phabricator: https://phabricator.wikimedia.org/T141281