Attempt a last-ditch ChannelManager persistence if the BP exits

[TheBlueMatt]

If the BP exits because it failed to write our ChannelManager to disk, we'll probably lose some channels on startup. However, there's still some chance for us to write the ChannelManager in a retry, which we should do before we exit.

[TheBlueMatt]

Oops, right.

[wpaulino]

Heh, one more time

-           e,
-           persist_res
+           e, persist_res

[TheBlueMatt]

Ugh, fixed.