Improve OpenAPI Documentation: error codes

[splitbrain]

Currently the getErrorCodes() method in the OpenAPIGenerator is used to create a simple list of error codes found in the APICore class. It is very simple and just uses a bunch of regular expressions.

There are different things that could be improved:

• use the PHP Tokenizer to parse the class
• also find error codes thrown in remote plugins
• maybe generate different response schemas for each registered call showing exactly the codes that a function might throw (this might be too complex as it would require a nearly complete static analysis)

Note: any implementation should not introduce any additional large dependencies to the code base. The simpler and more elegant the better.