Improve error message when channel manager cannot reach the site anymore

When the site is unreachable or down, the channel manager shows an error message after a while. However, that error message is rather incomprehensible for end users:

The connection to the HST timed out. Do you want to retry to establish a connection?

We should improve the message. And center it correctly in the popup dialog (see screenshot; it now seems to be aligned to the right?).

Steps to reproduce:
1. start GoGreen locally
2. login
3. open a channel in the channel manager
4. shutdown the site (cd site; mvn cargo:undeploy)
5. wait

Expected: the browser show a comprehensible error message after a while
Actual: the browser show an incomprehensible error message after a while