fix Issue 8960 - Unable to set thread priority

- Ignore error when get-/setting the priority of a terminated thread.
add missing nothrow @nogc to signal functions
require nothrow for callWithStackShell callback
Merge pull request #966 from MartinNowak/callWithStackShell

require nothrow for callWithStackShell callback
Merge pull request #550 from MartinNowak/fix8960

fix Issue 8960 - Unable to set thread priority