robustness improvements

[rakslice]

currently little is done to detect error conditions or help the user solve them.

• network connectivity
  ** after a notification that a new node is available, make sure that if the intiial http request times out or gets a (relevant) error, tell the user about a connectivity problem they may potentially need to fix
• disk full
  ** not sure where we're at with this; at least test it.
  ** add the ability to check free disk space, and in a disk space low condition, don't sync, and also don't save downloads that go through us. (it might make sense to have an option to just take the node offline if there are good nodes online)