fixup! Bug 31286: Implementation of bridge, proxy, and firewall settings in...

fixup! Bug 31286: Implementation of bridge, proxy, and firewall settings in about:preferences#connection

Bug 42299: Unbreak about:preferences#connection when invalid bridge
lines are supplied

We switched to a shared bridge line parser. It might throw if an invalid
bridge line is passed, but we do not handle the exception in
connectionPane.js. As a result, the page breaks.

As a workaround, we can simply ignore the errors, but a better solution
would warn the user about that.
A bridge card rework is expected to happen in the 13.5 cycle, so I think
we can defer a proper fix to that moment.
(This should also be the UX of 11.5, 12.0 and 12.5).