Fix to improve messaging and instructions around authentication.

Here's a fix for https://github.com/googlegenomics/api-client-r/issues/22.  Below is the output when auth is needed.  There's still room for improvement, but the timing of the auth reminder is good.
```
> library(GoogleGenomics)
> getReads()
Fetching reads page
 Error in checkResponse(response) : client error: (403) Forbidden 
6 stop(http_condition(x, "error", call = sys.call(-1))) 
5 stop_for_status(response) at client.R#94
4 checkResponse(response) at client.R#68
3 getSearchPage("reads", body, fields, pageToken) at reads.R#57
2 getReadsPage(readsetId = readsetId, chromosome = chromosome, 
    start = start, end = end, fields = fields, pageToken = pageToken) at reads.R#97
1 getReads() 
In addition: Warning messages:
1: In mapCurlOptNames(names(.els), asNames = TRUE) :
  Unrecognized CURL options: token
2: In checkResponse(response) :
  ERROR: Daily Limit for Unauthenticated Use Exceeded. Continued use requires signup.
Do not forget to authenticate.
Use GoogleGenomics::authenticate(file='secretsFile.json').
See method documentation on how to obtain the secretsFile.
```