[MOP] premature update_header run creates unresettable vtag

[mtbannister]

If update_header was run before Stephen has generated an astrometric solution, the error message is set to a long string that is the output of trying to read the default None object.
This string cannot be parsed and reset accurately by the current tag-reset that is done on rerunning update_header if the tag is not 'success'. Thus reruns of update_header that encounter these mis-set tags fail to run successfully.