WebUI: Channel does not exists in webui

[pcppcp]

Problem:
If webui cached state uses a saved state of a channel that has been closed already, only a server reply "Channel does not exist or has been closed(sender=0x0052d7b657553e7f47239d8c4431fef001a7f99c, open_block_number=3261525)" is displayed.
The message goes away only after deleting the cookies.

Solution:
The webUI should display either a button to forget/settle the channel or forget the saved data automagically.

[andrevmatos]

The webUI already provides those options, but they are, of course, only displayed when the webUI is served. As the signed balance proof is sent to the server through cookies, it persists across sessions, so the server thinks it is trying to send BP again for old/closed channel (which isn't the case).
Solution is for the proxy to serve the webUI even in the above exception, so the user has its options.

[pcppcp]

fixed