Provide additional response when external-plugin (cherrypick) fails

[deepsm007]

When using cherrypick plugin if there are any failures those needs to be surfaced on the PR with appropriate response. In this case when the diff for a PR is too large there is no response provided. Need to manually check the logs:

{"component":"cherrypicker","error":"failed to get patch: status code 406 not one of [200], body: {\"message\":\"Sorry, the diff exceeded the maximum number of files (300). Consider using 'List pull requests files' API or locally cloning the repository instead.\",\"errors\":[{\"resource\":\"PullRequest\",\"field\":\"diff\",\"code\":\"too_large\"}],\"documentation_url\":\"https://docs.github.com/rest/pulls/pulls#list-pull-requests-files\"}","event-GUID":"3567fe10-178d-11ef-8d06-fcff71c12be5","event-type":"issue_comment","file":"sigs.k8s.io/prow/cmd/external-plugins/cherrypicker/server.go:151"