Throwing subclasses of JS `Error` should be converted into classed R conditions (or similar)

[georgestagg]

When executing JS code from R, throwing a JS Error raises an R error condition:

> webr::eval_js("throw new Error('Something bad happened')")
Error in webr::eval_js("throw new Error('Something bad happened')") : 
  An error occurred during JavaScript evaluation:
  Something bad happened

We would like to be able to catch and handle such JS errors on the R side. This can be done currently by handling the R error condition and string matching on the Error's message text, but in principle we should be able to match on something constant in the presence of message translation.

I think from JS we should be able to throw subclasses of Error, and webR should be able to capture that information and convert it into classed R error conditions, or attach the JS class information to the R condition in some other way so that a particular type of error can be matched in R condition handlers.