Model Directives error function mechanism should be a post-loading transform

[ggould-tri]

(this code was in dev at the time this issue was filed but is moving out shortly; this issue is to tag a TODO in that process)

The model directives mechanism for incorporating user-provided error is ugly (the caller supplies an error function which is invoked by AddWeld on each invocation).

Simply removing the error function mechanism would be incorrect: Modeling error is a core source of error that has been missing and a major motivation for bringing model directives to drake. A superior mechanism would be to postprocess the parsed model directives with a similar error function (though using names rather than frame objects).

To this end #14038 will move the error function to using names rather than frames. This should allow error injection to be migrated to a directives preprocessing step within ProcessModelDirectives in the future without any API or semantics changes:

• As the first step of ProcessModelDirectives
• Walk the model directives, expanding names
• When an AddWeld is encountered that matches the error function, apply the error transform to the weld transform.
• Remove the frame name check from the unit test.

This preprocessing step can then be factored out into a separate post-loading transform step:

• Factor the code to handle the optional argument into a separate public function.
• Call that function with the optional argument.
• Deprecate the optional argument.

[ggould-tri]

Note that the ModelInstanceInfo structure might subsume some part of this mechanism. They serve related purposes, although that structure lacks the frame-level convenience of handling this in AddWeld.

[jwnimmer-tri]

FYI It's also possible that the ongoing work to use System Parameters on the plant would allow us to update weld joint offsets within the Context, which is even better and so the directives-time syntax for it would be unnecessary (or would simply feed into the default Parameter).

[sherm1]

Yes, I believe @joemasterjohn is working on that as we type.

[SeanCurtis-TRI]

Team assigned based on past assignment of the porting work (#13282). Feel free to change it if incorrect.

[jwnimmer-tri]

We've stopped using ModelWeldErrorFunction in Anzu. We should deprecate it within Drake.