Fix for bug 1225483

Fix for bug 1225483

(bug 1225483: "Error when trying to deploy without a network connection isn't user friendly.")

Running "juju deploy" without an internet connection fails in
CharmStore.Info(), so I modified the error handling of the HTTP
request issued there.

Two errors are possible: If a DNS lookup is necessary, net.DNSError
is raised; if the client machine locally caches DNS, net.OpError
is raised, so both errors are "translated" into the new error
message.

Testing the net.OpError case the naive way is time consuming: I considered
to use charm.NewStore("http://192.0.2.0"): The network 192.0.2.0/24
is a testing network, so access to this address indeed laed reliably
to an error, but it can be a timeout error -- after 30 seconds or so.
(Without an internet connection, one would get an immedieate "no route
to host" error). So I copped out from writing a specific test for this
case...

https://code.launchpad.net/~adeuring/juju-core/1225483/+merge/202699

(do not edit description out of merge proposal)

[abel.deuring, 2014-01-22 16:14:14]

Please take a look.

[natefinch, 2014-01-22 16:45:56]

LGTM with one slight fix.

https://codereview.appspot.com/55690043/diff/1/charm/repo.go
File charm/repo.go (right):

https://codereview.appspot.com/55690043/diff/1/charm/repo.go#newcode146
charm/repo.go:146: case *net.OpError:
You can combine these cases by making it:
case *net.DNSError, *net.OpError:

That way you don't have to duplicate the message.