[validation] add error if path do not start with a slash

Created by: jmini

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.3.x, 4.0.x. Default: master.
• Copied the technical committee: @OpenAPITools/generator-core-team @OpenAPITools/openapi-generator-collaborators (change in core)

Description of the PR

From https://github.com/OpenAPITools/openapi-generator/pull/967#issuecomment-418602467

This spec is not valid:

openapi: '3.0.1'
info:
  title: ping test
  version: '1.0'
servers:
  - url: 'http://localhost:8000/api/'
paths:
  ping/me:
    get:
      operationId: pingGet
      responses:
        '201':
          description: OK

Because the path ping/me do not conform to this rule:

The field name MUST begin with a slash.

See the OpenAPI Specification 3.0.1.

Some editors like Swagger online editor or the KaiZen-Editor are reporting errors, but Swagger-Parser do not.

This PR add additional errors to the Swagger-Parser error message. If the user decide to by-pass the validation he might get invalid generated code.