Optimize exceptions for nested objects

[wol-soft]

Currently validation errors for nested objects are handled identically to the errors of the parent object. This may lead to confusing errors:

{
  "id": "Person",
  "type": "object",
  "properties": {
    "name": {
      "type": "string",
    },
    "pet": {
      "type": "object",
      "properties": {
        "name": {
          "type": "string",
        },
        "age": {
          "type": "integer",
        }
      }
    }
  }
}

Initializing a object like new Person(['name' => false, 'pet' => ['name' => false]]); will throw an exception with the message:

Invalid type for name. Requires string, got boolean
Invalid type for name. Requires string, got boolean

If only one name validation fails the exception isn't helpful to detect whether the name of the main object isn't valid or the name of the nested object.

This PR adds a NestedObjectException wrapping exceptions from nested object instantiation. The NestedObjectException will additionally change the output behaviour of the exception so the exception for the example given above will look like:

Invalid type for name. Requires string, got boolean
Invalid nested object for property pet:
  - Invalid type for name. Requires string, got boolean

[coveralls]

Pull Request Test Coverage Report for Build 306

• 13 of 13 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 98.337%

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Totals
Change from base Build 304:	0.004%
Covered Lines:	1715
Relevant Lines:	1744

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💛 - Coveralls