Ignore "No Checksum with line number" errors as they trigger a resend just as well as e.g. "No Line Number with checksum" (quick check of the Marlin source seems to confirm this).

Should hopefully help with #5 and #9 -- will not reduce the communication errors (that's most probably not the comm stacks fault but rather a noise issue or something) but will lead to a more error resilient behaviour. Please test (I can't reproduce the issue(s)).