chore: update err msg for policy verification

[qweeah]

This PR rephrased error message returned during trust policy verification.

[patrickzheng200]

@qweeah Have you checked the following?

1. run go test -v . under trustpolicy dir. All tests need to be passed.
2. run the notation CLI e2e test again, because there are test cases expecting this error message. Changing the error message would fail the e2e test. All e2e tests need to be passed as well.

[qweeah]

Will update UT.

2. Changing the error message would fail the e2e test. All e2e tests need to be passed as well.

Shouldn't that be done when bumping notation-go? If I update e2e spec, then it will fail without the right version of notation-go.

[patrickzheng200]

2. Changing the error message would fail the e2e test. All e2e tests need to be passed as well.

Shouldn't that be done when bumping notation-go?

oh yeah, if you need to change the e2e, you could include the change in this PR: notaryproject/notation#593 because this change is connected closely with your CLI trust policy command.
Merge the notation-go PR first -> go mod tidy on notation CLI side -> merge the notation CLI PR.

[patrickzheng200]

LGTM

[patrickzheng200]

LGTM

[priteshbandi]

LGTM

[shizhMSFT]

LGTM

[shizhMSFT]

I'm not able to merge this PR due to

caused by recent change by notaryproject/notation-core-go#135

[shizhMSFT]

Updated the branch policy to match the workflow defined in notation-core-go.

[qweeah]

@qweeah Have you checked the following?

1. run go test -v . under trustpolicy dir. All tests need to be passed.
2. run the notation CLI e2e test again, because there are test cases expecting this error message. Changing the error message would fail the e2e test. All e2e tests need to be passed as well.

@patrickzheng200 I bumped notation-go including this change and ran an E2E in my own fork, looks like no test will fail thus no further change required on CLI side.