Improve diagnostics for let_value case

[maikel]

Diagonstics for

 stdexec::sync_wait(stdexec::let_value(stdexec::just(), [](int) { return stdexec::just(); }));

is good. Diagnostics for the following erronous code is bad

 stdexec::sync_wait(stdexec::let_value(stdexec::just(), [] { return ; }));

I expect an error message along the lines "The returned type of the function is not a sender"