Preserve decorator state across webapi decorators.
Several of the webapi decorators attempted to set things like possible

error results, but they'd be lost, due to decorators not passing on that

information correctly. In some other cases, the wrong function would be

modified (which ended up working anyway, due to the function's

dictionary being transferred after, but it polluted the function).
There's now a @webapi_decorator, which intelligently merges things like

the error response and request field lists. It's meant as a replacement

for @simple_decorator for creating webapi-specific decorators.
Along with this, the calls to @webapi_response_errors in decorators have

been moved to decorate the correct function.

Added new unit tests, which pass.
Tested with my API lintian tool that error responses were being set correctly

now.