Improve error message ERROR: not_found when credential identifier is not found

[bryankaraffa]

An error is raised when the credential identifier provided does not exist in the Org/Project. Unfortunately this error is nondescript and new users of fpt often get stuck without knowing how to fix the issue

Here's an example of the error today:

ftp run policy.pt --credential "auth_aws=This_Credential_Does_Not_Exist"
ERROR: not_found

Ideally the error would be more descriptive to point the user to the issue:
ERROR: Credential with identifier "This_Credential_Does_Not_Exist" not found