Add sourcemaps to some swagger validation errors

[kylef]

I noticed source map was missing for one particular error message:

error: (4) Validation failed. Property 'authorizerId' listed as required but does not exist in '/definitions/authorizerInfoType'

The error was as follows:

{
  stack: "SyntaxError: Validation failed. Property 'authorizerId' listed as required but does not exist in '/definitions/authorizerInfoType'\n" +
    '    at validateRequiredPropertiesExist (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/swagger-parser/lib/validators/spec.js:338:19)\n' +
    '    at validateSpec (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/swagger-parser/lib/validators/spec.js:37:5)\n' +
    '    at SwaggerParser.validate (/Users/kyle/Projects/apiaryio/api-elements.js/node_modules/swagger-parser/lib/index.js:172:7)',
  message: "Validation failed. Property 'authorizerId' listed as required but does not exist in '/definitions/authorizerInfoType'",
  toJSON: [Function: toJSON],
  name: 'SyntaxError',
  toString: [Function: toString]
}

Given the JSON path is in the error message I was able to extract it out and attach sourcemaps via it.

[opichals]

Cool!