Issue/228 cmdstan retcodes

[mitzimorris]

Submission Checklist

• Builds locally
• Declare copyright holder and open-source license: see below

Summary

add section on error codes to CmdStan users guide, also appendix section.

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Columbia University

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)