feat: add errorMessage from schema when the property is missing

[p-spacek]

What does this PR do?

adds an errorMessage from the schema when the property is missing
allows customization of the errorMessage in more complicated scenarios, like

anyOf: [
          {
            required: ['title'],
            errorMessage: 'At least one of `title` or `icon` must be defined.',
          },
          {
            required: ['icon'],
          },
        ],

What issues does this PR fix or reference?

Is it tested? How?

test

[coveralls]

coverage: 83.961% (+0.005%) from 83.956%
when pulling 4977373 on jigx-com:feat/add-errorMessage-from-schema-when-the-property-is-missing
into 410cff9 on redhat-developer:main.

[datho7561]

Does any other YAML/JSON validation service support this? It seems like an odd thing for us to introduce in the yaml-language-server unless it's in the JSON Schema specification or there's an established convention for doing this.

Yes, VS Code's JSON language server supports it.

I can see that vscode-json-languageservice has it listed here: https://github.com/microsoft/vscode-json-languageservice/blob/e0f39f09a3e693f3a0895a051efd4895cbc6a746/src/jsonSchema.ts#L78, but I can't seem to get it to actually show the message I set when working with JSON in VS Code.

Nevermind, got it working in VS Code. The specific scenario I tried initially didn't work properly, though.

Also, it seems we already support errorMessage for many scenarios, my bad.

[datho7561]

Looks good. Thanks, Petr!