ChannelForm: Show message intead of stacktrace when no channel type found

[sukhwinder33445]

fixes #225

Requires:

• Remove .action-link and button-link css icingaweb2#5370
• ButtonLink: Allow to disable the button ipl-web#284

[ncosta-ic]

@sukhwinder33445 I feel like it looks kinda weird with the current implementation, as it still shows the name input, even though there's no submit button or the like.

What if you moved the result check to the top of the assemble method? I think this would correlate with our philosophy of having early breakouts if required data is not provided to a method.

It currently looks like this:

And adding the check and early return to the top of the method would result in this representation:

The displayed message is also missing a dot (.) at the end of the second sentence.

Keep in mind that I only wondered about the reasoning for this exact implementation, this isn't really a suggestion. But if you do feel like it would improve the UI, feel free to change it. Will do a proper review after your comment on this topic :)

[nilmerg]

Our actual philosophy on this topic is not allowing access to a form at all. i.e. to disable the button that leads to it and adding a title to explain why.

[nilmerg]

…or to hide the button but still explain why if there is no contextual hint.