Display error message if internet not working.
Display error message if internet not working.

An error message with 'Try again' option is displayed
if the internet is not working.
Fixes #114

zulip/zulip-electron,zulip/zulip-desktop,brainwane/zulip-electron,zulip/zulip-desktop,zulip/zulip-electron,brainwane/zulip-electron,zulip/zulip-electron,zulip/zulip-desktop,brainwane/zulip-electron,zulip/zulip-desktop