Can confirm this does actually work as expected, when manually logging in we get back a pitiful error from one password:
[changed@changed-desktop] /tmp/terraform-provider-onepassword/0.5.5$ ./op signin changed
Enter the password for changed@changed.org at changed.1password.com:
[LOG] 2020/05/01 12:00:44 (ERROR) 445:
[changed@changed-desktop] /tmp/terraform-provider-onepassword/0.5.5$
Can confirm this does actually work as expected, when manually logging in we get back a pitiful error from one password: