validator: return validation error for extension validation errors

[christopher-angel]

Description

When validating schema extensions the validator result isn't very helpful. Here is an example error:

validation failed: jsonschema: '' does not validate with https://stac-extensions.github.io/raster/v1.1.0/schema.json#/oneOf: oneOf failed

It would be useful to receive the report for the entire validation, especially the failed resource. With this change we should see the full validation result - example:

invalid collection: /path/to/failed/collection.json
[I#] [S#] doesn't validate with https://stac-extensions.github.io/raster/v1.1.0/schema.json#
  [I#] [S#/oneOf] oneOf failed
    [I#] [S#/oneOf/0/allOf/0] allOf failed
      [I#] [S#/oneOf/0/allOf/0]
        [I#] [S#/oneOf/0/allOf/0/required] missing properties: 'assets'
        [I#/type] [S#/oneOf/0/allOf/0/properties/type/const] value must be "Feature"
    [I#] [S#/oneOf/1/allOf/0] allOf failed
      [I#/item_assets/acd] [S#/oneOf/1/allOf/0/properties/item_assets/additionalProperties/$ref] doesn't validate with '/definitions/assetfields'
        [I#/item_assets/acd/raster:bands] [S#/definitions/assetfields/properties/raster:bands/$ref] doesn't validate with '/definitions/bands'
          [I#/item_assets/acd/raster:bands/0/nodata] [S#/definitions/bands/items/properties/nodata/oneOf] oneOf failed
            [I#/item_assets/acd/raster:bands/0/nodata] [S#/definitions/bands/items/properties/nodata/oneOf/0/type] expected number, but got string
            [I#/item_assets/acd/raster:bands/0/nodata] [S#/definitions/bands/items/properties/nodata/oneOf/1/enum] value must be one of "nan", "inf", "-inf"

This is accomplished by ensuring that the extension error is actually a jsonschema.ValidationError and then pass this to the newValidationError func to preserve context.

If for some reason we can't parse the error as expected, we go ahead and return it as usual.

[tschaub]

Thanks, @christopher-angel