Fix additionalProperties error with errorDataPath

[lehni]

As discussed with @epoberezkin in #671 (comment), here now the PR to add correct error messages for additionalProperties validation errors when errorDataPath is set to 'property'

[lehni]

This probably should need a test for it too, but I couldn't figure out where to put it, and couldn't find a similar one for the same behavior for required here.

[epoberezkin]

@lehni well spotted the test was missing for the other change. The tests should be somewhere here https://github.com/epoberezkin/ajv/blob/797dfc8c2b0f51aaa405342916cccb5962dd5f21/spec/errors.spec.js#L45 and here https://github.com/epoberezkin/ajv/blob/797dfc8c2b0f51aaa405342916cccb5962dd5f21/spec/errors.spec.js#L145 (for "required" errors).

At the moment it tests only dataPaths, should also check messages.

[lehni]

@epoberezkin thanks for the pointer. Here now my added tests:

4a12ca3#diff-831dd6f83c36a271e002b7770f3dbac9R67

I hope these suffice?

Also, please let me know if 'is an invalid additional property' sounds like a good message to you. The wording can change. Alternatives to invalid are illegal or unsupported

[epoberezkin]

@lehni thank you. I've merged. Maybe just "is an additional property"?

[lehni]

I think the message should somehow state that additional properties aren't allowed. should NOT have additional properties implied that. But how to phrase it for the message that is directly attached to the property?