Fix two Passphrase credential type issues

Summary:
Fixes T4991. Two issues:

• These error messages pass an object to "%s", when they mean to pass a type constant.
• The check for noncreatable credentials is incorrectly in the "edit" branch of the controller.

Test Plan:

• Edited a "SSH Key on disk" credential.
• Tried to create a credential with a bogus type.

Reviewers: btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T4991

Differential Revision: https://secure.phabricator.com/D9299