Enable file_safety_check in new diagnostics

• Enable file_safety_check in new diagnostics

  Reuse subroutine across old and new diagnostics

• Improve error messages if restart files are not writable

  • Include actual name of restart directory

  • Include error message from system

  Note that the system error message may not be very useful in the case where the directory doesn't exist, because we're actually checking we can write a file. Fortran doesn't seem to have a portable, intrinsic method of checking directories

Previously, check_restart_file_writeable was implemented in the new diagnostics but the call to it was commented out in 9381fdd. The reason why doesn’t seem to have been recorded.