Add annotation to backtraces for practicality.

[ksss]

Exception#backtrace may return nil.

Exception.new.backtrace #=> nil

However, in practical use cases, backtraces are almost never nil in the vast majority of situations.

begin
  ...
rescue => e
  logger.warn(e.backtrace.join("\n"))

  loc = ex.backtrace_location.first
  loc.lineno
end

Similar to other overloads marked with %a{implicitly-returns-nil}, I propose adding %a{implicitly-returns-nil} to backtraces as well, placing emphasis on practicality.