Improve error handling in verifyConn

[ribbybibby]

• Avoid a panic when the http.Get resp is nil by returning the error before deferring resp.Body.Close()
• http.Get will not return an error for non-2xx response codes, so check the response code explicitly and return an error when it isn't the expected 200 response
• Return the error from ioutil.ReadAll
• Use fmt.Errorf across the board
• Include the caURL in the error messages to hopefully make the source of errors clearer up the stack