No feedback for offline source

[mtib]

When the source is offline, the user should be asked if he wanted to proceed, wait or cancel.
It'd be fine if I'd throw an exception if the source is offline. The problem is that the connection is made in GrADS, because vanilla-python does not handle OpenDap.
To solve this I either would need to add a lot of GrADS logic, or implement the visualization myself with PIL and PyDap. I am torn.