Capture errors in datasets() if credentials are incorrect.

[andrie]

If the user provides invalid credentials, then the first indication of this might be when datasets() is called.

For example:

> ws <- workspace(id = "blah")
> datasets(ws)
Error in if (is.null(x) || is.na(x$Name[1])) { : 
  missing value where TRUE/FALSE needed
In addition: Warning message:
In is.na(x$Name[1]) :
  is.na() applied to non-(list or vector) of type 'NULL'

Provide better error capturing for:

• datasets()
• services()
• experiments()

[andrie]

We should run datasets() at the end of the construction of the Workspace object.

Note: add an argument .validateAPI = TRUE, so we can construct the object without validation. This will be necessary for testing purposes.