update: improve missing trustpolicy error message

[kody-kimberl]

This PR improves the error message for a missing trustpolicy file

This PR intends to resolve the last comment in the following issue:
notaryproject/notation/#128

This is the output when the trustpolicy is missing:

c889f3b9d811:notation kodysk$ ./bin/notation verify $IMAGE
Warning: Always verify the artifact using digest(@sha256:...) rather than a tag(:v1) because resolved digest may not point to the same signed artifact, as tags are mutable.
Error: Trust policy is not present, please create trust policy at /Users/kodysk/Library/Application Support/notation/trustpolicy.json

Signed-off-by: Kody Kimberl [email protected]

[kody-kimberl]

I made a new commit that addresses the above comments. It now checks the error type to determine what message to return.

Output when trustpolicy is missing:

c889f3b9d811:notation kodysk$ ./bin/notation verify $IMAGE
Warning: Always verify the artifact using digest(@sha256:...) rather than a tag(:v1) because resolved digest may not point to the same signed artifact, as tags are mutable.
Error: trust policy is not present, please create trust policy at /Users/kodysk/Library/Application Support/notation/trustpolicy.json

Output when trustpolicy has 000 permissions:

c889f3b9d811:notation kodysk$ ./bin/notation verify $IMAGE
Warning: Always verify the artifact using digest(@sha256:...) rather than a tag(:v1) because resolved digest may not point to the same signed artifact, as tags are mutable.
Error: unable to read trust policy due to file permissions, please verify the permissions of /Users/kodysk/Library/Application Support/notation/trustpolicy.json

[priteshbandi]

LGTM

[patrickzheng200]

LGTM

[JeyJeyGao]

LGTM