Allow for configuring validation errors by paths

**What version of Ajv you are you using?**

`ajv@6.10.0`

**What problem do you want to solve?**

Let's say that the JSON Schema used to verify a value uses `oneOf` _(or something similar where multiple schemas are consulted, and may each have N reasons to fail validation)_ and a single mistake in the data results in N unique validation errors for each schema in the `oneOf`. A real-world example:

* Data: https://github.com/OAI/OpenAPI-Specification/blob/master/examples/v2.0/json/petstore.json
* JSON Schema: https://github.com/OAI/OpenAPI-Specification/blob/master/schemas/v2.0/schema.json

If I change line 32 in the `petstore.json` from `query` to `elsewhere`, the `oneOf` on line `925` of `schema.json` results in multiple errors:

```json
[
{
"keyword": "additionalProperties",
"dataPath": "/paths/~1pets/get/parameters/0",
"schemaPath": "#/additionalProperties",
"params": {
"additionalProperty": "type"
},
"message": "should NOT have additional properties"
},
{
"keyword": "enum",
"dataPath": "/paths/~1pets/get/parameters/0/in",
"schemaPath": "#/properties/in/enum",
"params": {
"allowedValues": [
"header"
]
},
"message": "should be equal to one of the allowed values"
},
{
"keyword": "enum",
"dataPath": "/paths/~1pets/get/parameters/0/in",
"schemaPath": "#/properties/in/enum",
"params": {
"allowedValues": [
"formData"
]
},
"message": "should be equal to one of the allowed values"
},
{
"keyword": "enum",
"dataPath": "/paths/~1pets/get/parameters/0/in",
"schemaPath": "#/properties/in/enum",
"params": {
"allowedValues": [
"query"
]
},
"message": "should be equal to one of the allowed values"
},
{
"keyword": "enum",
"dataPath": "/paths/~1pets/get/parameters/0/required",
"schemaPath": "#/properties/required/enum",
"params": {
"allowedValues": [
true
]
},
"message": "should be equal to one of the allowed values"
},
{
"keyword": "oneOf",
"dataPath": "/paths/~1pets/get/parameters/0",
"schemaPath": "#/oneOf",
"params": {
"passingSchemas": null
},
"message": "should match exactly one schema in oneOf"
},
{
"keyword": "oneOf",
"dataPath": "/paths/~1pets/get/parameters/0",
"schemaPath": "#/oneOf",
"params": {
"passingSchemas": null
},
"message": "should match exactly one schema in oneOf"
},
{
"keyword": "additionalProperties",
"dataPath": "/paths/~1pets/get/parameters/0",
"schemaPath": "#/additionalProperties",
"params": {
"additionalProperty": "name"
},
"message": "should NOT have additional properties"
},
{
"keyword": "oneOf",
"dataPath": "/paths/~1pets/get/parameters/0",
"schemaPath": "#/items/oneOf",
"params": {
"passingSchemas": null
},
"message": "should match exactly one schema in oneOf"
}
]
```

**What do you think is the correct solution to problem?**

In the example above, there is only one problematic path _(`/paths/~1pets/get/parameters/0`)_ and it would be nice for a tooling author to identify this. _(This might be specific to `oneOf` and similar, something to think about.)_

It would be nice to have an option to allow the grouping of errors by path so that you can report a singular error, like _"Not a valid parameter definition"_, instead of each individual reason as to why the singular data path is invalid. [z-schema](https://github.com/zaggino/z-schema) does this as well, by putting the individual reasons into an error property called `inner`. `z-schema` actually groups `oneOf` errors into its own group of failing errors. Below is the same document validated with `z-schema` _(for reference)_:

```json
[
{
"code": "ONE_OF_MISSING",
"params": [],
"message": "Data does not match any schemas from 'oneOf'",
"path": [
"paths",
"/pets",
"get",
"parameters",
"0"
],
"schemaId": "http://swagger.io/v2/schema.json#",
"inner": [
{
"code": "ONE_OF_MISSING",
"params": [],
"message": "Data does not match any schemas from 'oneOf'",
"path": [
"paths",
"/pets",
"get",
"parameters",
"0"
],
"inner": [
{
"code": "OBJECT_MISSING_REQUIRED_PROPERTY",
"params": [
"schema"
],
"message": "Missing required property: schema",
"path": []
},
{
"code": "ONE_OF_MISSING",
"params": [],
"message": "Data does not match any schemas from 'oneOf'",
"path": [],
"inner": [
{
"code": "ENUM_MISMATCH",
"params": [
"elsewhere"
],
"message": "No enum match for: elsewhere",
"path": [
"in"
],
"description": "Determines the location of the parameter."
},
{
"code": "ENUM_MISMATCH",
"params": [
"elsewhere"
],
"message": "No enum match for: elsewhere",
"path": [
"in"
],
"description": "Determines the location of the parameter."
},
{
"code": "ENUM_MISMATCH",
"params": [
"elsewhere"
],
"message": "No enum match for: elsewhere",
"path": [
"in"
],
"description": "Determines the location of the parameter."
},
{
"code": "ENUM_MISMATCH",
"params": [
false
],
"message": "No enum match for: false",
"path": [
"required"
],
"description": "Determines whether or not this parameter is required or optional."
}
]
}
]
},
{
"code": "OBJECT_MISSING_REQUIRED_PROPERTY",
"params": [
"$ref"
],
"message": "Missing required property: $ref",
"path": [
"paths",
"/pets",
"get",
"parameters",
"0"
]
}
]
}
]
```

**Will you be able to implement it?**

Sure!