Make WebTransportError constructor compatible

[achristensen07]

Chrome seems to be currently using this:
constructor(optional WebTransportErrorInit init = {});
This allows construction with the first parameter being a dictionary, like this example from wpt:
new WebTransportError({streamErrorCode: WT_CODE}))
The current spec IDL looks like this:
constructor(optional DOMString message = "", optional WebTransportErrorOptions options = {});
This says to interpret the first parameter as a string, and if it's not a string then use the empty string, which isn't compatible with wpt or Chrome. I think we should change it to something like this:

constructor(optional WebTransportErrorOptions options = {});
constructor(DOMString message, optional WebTransportErrorOptions options = {});

This would allow us to have a dictionary as the only parameter or to have a string or a string followed by a dictionary. I'm implementing this in the initial version of WebKit/WebKit#56176

[jan-ivar]

Thanks Alex for catching this! I see the spec's new WebTransportError("hello", {streamErrorCode: 2}) (pasted to web console) works only in Safari (w/feature flag). Both Chrome and Firefox TypeError. We forgot to update WPT in #425.

[jan-ivar]

What's your thought on the following which produces "hello" in Chrome and Firefox, but "[object Object]" in Safari?

new WebTransportError({streamErrorCode: 2, message: "hello"}).message

Ofc if we were to add that as well, then we'd need to decide what this should produce, so perhaps better to leave it out?

new WebTransportError("foo", {streamErrorCode: 2, message: "bar"}).message

[achristensen07]

I was thinking we also have the option of having the message parameter second, but I like having message as part of the dictionary and having no separate message parameter even better.

[jan-ivar]

We have a couple of options, but I think I still prefer your first solution, since it complies with WebIDL guidance:

constructor(optional WebTransportErrorOptions options = {});
constructor(DOMString message, optional WebTransportErrorOptions options = {});

[achristensen07]

I obviously don't have a strong opinion here. WebKit has 3 classes that inherit from DOMException:
RTCError, which doesn't follow the WebIDL guidance
GPUPipelineError, which does follow the WebIDL guidance and takes the dictionary as the second parameter and string as the first parameter and uses the empty string if the first parameter is anything but a string
WebTransportError, which mostly follows WebIDL guidance with this proposal