disallow negative `total_deposit` when patching a channel

[pcppcp]

API should return an error if user attempts to deposit negative amount.
PR w/ fix & test will follow soon.

[pcppcp]

@kelsos could you also update webui to disallow entering negative amounts when doing channel deposits? Maybe on transfers as well.

[kelsos]

@pcppcp I will give it a look and see if a check is in place. This issue, however, had another cause more tricky in nature.

I had open a channel with 10 tokens deposit, and I tried to deposit another 5. The channel had 10 tokens but the value was not properly updated to the webui. When I tried to increase my deposit by another 5, due to the reported deposit being 0 the webui send a total deposit of 5 where is should send 15. So it was not an actual negative total deposit, but the newer total deposit was lower than the previous resulting in the error.