closesTryGhost/Support#426
refs TryGhost/gscan#106
needs TryGhost/gscan#107
GScan can return errors, which was not handled in our theme validator and caused Ghost to crash completely. GScan will now return an Ignition error when its not able to read the `.zip` file.
e. g.: `{"errors":[{"message":"Failed to read zip file","context":"tife.zip","errorType":"ValidationError","errorDetails":"invalid relative path: ../tife/"}]}`