Incorrect error handling when a step includes multiple attach workflow commands

Current situation

If a step contains more than one ::attach:: workflow command, if one of the first fails the error is not properly handled.

In particular, the following

- run: |
    echo "::attach name=foo::yada"
    echo "::attach name=bar::yidi"
  continue-on-error: true

will fail if neither yada nor yidi exist.