Add nicer error message when failing to verify rekor2 entries

[jku]

Description

sigstore-js does not yet understand rekor2 entries, but those are now starting to show up (multiple clients already sign with rekor2 on staging). Would be good if current sigstore-js had a decent error message when sigstore-js is used to verify a signature bundle that contains a rekor2 entry.

Current error:

/usr/local/lib/node_modules/@sigstore/cli/node_modules/@sigstore/verify/dist/timestamp/checkpoint.js:50
        throw new error_1.VerificationError({
              ^

VerificationError: invalid checkpoint signature
    at verifyCheckpoint (/usr/local/lib/node_modules/@sigstore/cli/node_modules/@sigstore/verify/dist/timestamp/checkpoint.js:50:15)
    at verifyTLogTimestamp (/usr/local/lib/node_modules/@sigstore/cli/node_modules/@sigstore/verify/dist/timestamp/index.js:26:43)
    at /usr/local/lib/node_modules/@sigstore/cli/node_modules/@sigstore/verify/dist/verifier.js:55:64
    at Array.map (<anonymous>)
    at Verifier.verifyTimestamps (/usr/local/lib/node_modules/@sigstore/cli/node_modules/@sigstore/verify/dist/verifier.js:48:46)
    at Verifier.verify (/usr/local/lib/node_modules/@sigstore/cli/node_modules/@sigstore/verify/dist/verifier.js:35:33)
    at Object.verify (/usr/local/lib/node_modules/@sigstore/cli/node_modules/sigstore/dist/sigstore.js:108:22)
    at /usr/local/lib/node_modules/@sigstore/cli/node_modules/sigstore/dist/sigstore.js:81:64
    at async Verify._run (/usr/local/lib/node_modules/@sigstore/cli/node_modules/@oclif/core/lib/command.js:181:22)
    at async Config.runCommand (/usr/local/lib/node_modules/@sigstore/cli/node_modules/@oclif/core/lib/config/config.js:456:25) {
  code: 'TLOG_INCLUSION_PROOF_ERROR',
  cause: undefined
}