Improve error handling

[jettdc]

Errors are currently hard to catch, they’re not strongly typed so I had to let my code throw the error first, then find the name, then add an catch clause to check for that error name

Being able to do instance checking would be much nicer

if isinstance(e, ResourceNameAlreadyExistsError):
    ...

or at least an enum of error types per namespace:

if e.name == core.Errors.RESOURCE_NAME_ALREADY_EXISTS:
    ...

or something like:

if isResourceNameAlreadyExistsException(e):
    ...

[jsmith]

There is ongoing work to improve error handling in both the Python SDK but also the API in general. I'll keep this FR up-to-date when this feature has been released. We'll likely be going with the first option where we subclass PalantirRPCException for each error.