fix: raise error after output collected

[beckermr]

Guidelines and Ground Rules

• Don't migrate more than several hundred feedstocks per hour.
• Make sure to put [ci skip] [skip ci] [cf admin skip] ***NO_CI*** in any commits to
  avoid massive rebuilds.
• Rate-limit commits to feedstocks to in order to reduce the load on our admin webservices.
• Test your migration first. The https://github.com/conda-forge/cf-autotick-bot-test-package-feedstock is available to help test migrations.
• GitHub actions has a GITHUB_TOKEN in the environment. Please do not exhaust this
  token's API requests.
• Do not rerender feedstocks!

Items 1-3 are taken care of by the migrations code assuming you don't make any significant changes.