Safe pythonic access to newly created agents

[stonier]

• Make sure python has a handle (pointer) to the agent after it is passed to the simulator so it can manipulate custom properties of that agent (e.g. the nominal speed)
• AddAgent should be a bit better at error handling - right now we have c++ coverage! assert, exceptions and return codes.

[stonier]

Error Handling

Prefer not to have asserts in an underlying library. It takes away choice from the user of the library. Exceptions or return codes - I'm fine either way. Maybe a group question on slack to canvas opinions on how we'd generally like to do error handling in the backend?

[stonier]

Some python questions here - how it is handling unique pointers and exceptions?

[clalancette]

I talked to @basicNew and @hidmic about this for a bit today. Here's a quick summary of our discussion:

1. @basicNew and @hidmic prefer to do exceptions.
2. The google style guide disallows exceptions. However, this is largely due to legacy concerns.
3. The drake style guide allows exceptions, but not catch. They essentially function as fancy asserts.
4. @hidmic suggests that drake internally would not be able to recover if one of our systems did something badly, and thus throwing exceptions in those cases would not be useful (since the application as a whole probably cannot continue).
5. After some discussion, we came to the conclusion that we should use DELPHYNE_DEMAND for "internal" or drake-related errors, and use exceptions when the user of an API provides invalid arguments.

@stonier Does that sound like a reasonable plan to you?

[clalancette]

After further discussion, we are going to go the drake route of using throw, but not catch. This essentially gives us more fancy asserts, with the possibility of handling them later. PR upcoming.

[clalancette]

This is done as of #502, so closing for now.