docker trust key load fails without error on invalid file. #2031

Description
Using the command docker trust key load will fail but indicate "Successfully imported key from ..."

Steps to reproduce the issue:

1. Create a delegation key using the command: docker trust key generate key-name
2. Copy the newly generated private key file from ~/.docker/trust/private to a new user account or new machine.
3. Execute the command: "docker trust key load --name jeff --name new-key-id.key

Describe the results you received:
The program prompted:

Enter passphrase for encrypted key:
Successfully imported key from new-key-id.key

Inspection of the ~/.docker/trust/private directory shows the key was NOT imported.

Describe the results you expected:

If the key could not be imported, a useful error message should be displayed. Something like:
"Key key-name could not be imported. Was this key created using notary key export?"

Additional information you deem important (e.g. issue happens only occasionally):

The correct sequence of events is to EXPORT the key using the notary key export command. This adds the additional field "path" to the exported key file, which is evidently required.

notary key export --key key-id -o key-name.pem

Output of docker version:

Client:
 Version:           18.09.6
 API version:       1.39
 Go version:        go1.10.8
 Git commit:        481bc7715621
 Built:             Fri Jun  7 12:00:00 2019
 OS/Arch:           linux/amd64
 Experimental:      false

Output of docker info:

N/A

Additional environment details (AWS, VirtualBox, physical, etc.):
OpenSUSE Leap 15.1