Improve error messages for notation CLI

[nelson-wu]

Some examples:

$ notation cert generate-test
2021/11/12 13:53:30 missing certificate hosts

Perhaps flesh this out more with examples, similar to az cli

> az acr import -n 
the following arguments are required: --source

TRY THIS:
az acr import --name MyRegistry --source docker.io/library/hello-world:latest --image targetrepository:targettag
Import an image from a public repository on Docker Hub. The image uses the specified repository and tag names.

az acr import --name MyRegistry --source sourcerepository:sourcetag --image targetrepository:targettag --registry /subscriptions/00000000-0000-0000-0000-000000000000/resourceGroups/sourceResourceGroup/providers/Microsoft.ContainerRegistry/registries/sourceRegistry
Import an image from an Azure container registry in a different subscription.

https://docs.microsoft.com/en-US/cli/azure/acr#az_acr_import
Read more about the command in reference docs

[dtzar]

We should get better error messages as a result of moving to cobra, but should evaluate the experience of error messages after the PR merges #255

[dtzar]

Let's also in this improvement make sure the CLIs exit with proper exit code (non zero for failures) https://github.com/urfave/cli/blob/master/docs/v2/manual.md#exit-code

[iamsamirzon]

@dtzar - I created a new issue related to helping users deal with CLI errors. Refer #300

[priteshbandi]

Some examples of inappropriate errors:

Missing trustpolicy.json file

➜  notation git:(main) ✗ ./notation verify $IMAGE
Resolved artifact tag `v1` to digest `sha256:36ca4d6834ed680362327811238b97c687e77c5cf4a04a74d0853d3c0c17e059` before verification.
Warning: The resolved digest may not point to the same signed artifact, since tags are mutable.
Error: open /Users/pritesb/Library/Application Support/notation/trustpolicy.json: no such file or directory

Empty trustpolicy.json file

➜  notation git:(main) ✗ ./notation verify $IMAGE
Resolved artifact tag `v1` to digest `sha256:36ca4d6834ed680362327811238b97c687e77c5cf4a04a74d0853d3c0c17e059` before verification.
Warning: The resolved digest may not point to the same signed artifact, since tags are mutable.
Error: EOF

trustpolicy.json file content: {}

➜  notation git:(main) ✗ ./notation verify $IMAGE
Resolved artifact tag `v1` to digest `sha256:36ca4d6834ed680362327811238b97c687e77c5cf4a04a74d0853d3c0c17e059` before verification.
Warning: The resolved digest may not point to the same signed artifact, since tags are mutable.
Error: trust policy document uses unsupported version ""

trustpolicy.json file content: hola

➜  notation git:(main) ✗ ./notation verify $IMAGE
Resolved artifact tag `v1` to digest `sha256:36ca4d6834ed680362327811238b97c687e77c5cf4a04a74d0853d3c0c17e059` before verification.
Warning: The resolved digest may not point to the same signed artifact, since tags are mutable.
Error: invalid character 'h' looking for beginning of value