Define exceptions to be thrown in startSession and joinSession rejection about presentation-api

[anssiko]

I believe we should reuse the DOMException types, if the API just needs a way to represents an error, with no additional data (such as in the MediaStreamError's case, constraintName) beyond the optional message that can be used to convey user agent-defined human readable details of the error for debugging purposes.

For example, we could simply throw NotSupportedError in both the cases.

I'll tag this for F2F discussion, since this should be a relatively quick one.

[mfoltzgoogle]

I would also be in favor of re-using DOMException in the spec and using the human-readable message to provide the exact issue to the developer.

Unless there are arguments otherwise, we'll close this issue when the spec has been updated to throw DOMException types for error cases.

[anssiko]

@mfoltzgoogle Sounds good. I'd consider this a minor change that aligns the spec with the platform conventions.

Please feel free to submit a PR with "Fixes #78" in the commit message and merge it. That should automatically close the issue (thanks to https://help.github.com/articles/closing-issues-via-commit-messages/).

If any of the group participants have concerns with the change, we can always reopen the issue.