Catch `error` fields in esummary json files

[dwinter]

e.g., when an overly-large request is made (this currently returns and empty esummary record, and does not let the user know why)

{
    "header": {
        "type": "esummary",
        "version": "0.3"
    },
    "error": "Too many UIDs in request. Maximum number of UIDs is 500 for JSON format output."
}

We should use the existing JSON error checking functions to ensure these messages are passed on to the user.