Missing [[noreturn]] attribute in throw_exception() function

[vagran]

The throw_exception() function should have [[noreturn]] attribute.

My problem example is that I use canary library which calls this function. The calling function is declared as [[noreturn]] (which is reasonable), and missing [[noreturn]] in asio causes compiler warning ‘noreturn’ function does return. More sad is that this specific warning cannot be suppressed in GCC by command line option or pragma, so it makes it impossible to compile a project with canary dependency with "-Werror".