Fix #222
Fix #222

These chages fix issue #222.  The `validate_file` checks for each row were returning early if no file was found regardless of how the `allow_missing` flag was set.  The existing `allow_missing` check was after the early return.

The logic behind checking for a valid file has been updated to ensure the state of `allow_missing` is checked before returning early.

I have tested this locally and it works as intended now, my only concern is the messaging set to stderr in the `validate_file` funciton might be a bit confusing for anyone useing the code with this flag set as it still says `Error:` at the start.

I am happy to update the message to be a bit more dynamic and change to a `Warning:` when `allow_missing` is used, but was on the fence if the extra logic needed for that was really worth it in this case.

zooniverse/panoptes-cli