Better handle ftp errors in fw_update

Better handle ftp errors in fw_update

Trap STDERR to post-process it looking for 404 errors to handle them differently.

The fetch method now also returns different error codes for errors that can
continue on.  Currently only 404 is special and everything else should cause
fw_update to exit early without trying all the files.

Exit early if the SHA256.sig gets a 404 because that is required to figure out
what valid firmware are.