`transform_completion_signatures` should be allowed to return "error" types

[ericniebler]

transform_completion_signatures is specified to always return an instance of the completion_signatures class template. in stdexec, propagating error types through transform_completion_signatures is a part of the diagnostics story. some accommodation should be made for that.

[lewissbaker]

@ericniebler Is this issue addressed by P3164R0?
Or is there still more to do in addition to that paper?

[ericniebler]

implementing this in stdexec uncovered some corners that aren't yet addressed by P3164. i prefer to leave this issue open for now.