fail harder if we can't write announcement-seqnum

[meejah]

If the file announcement-seqnum in the node dir isn't writable, the code in write_config (in Node, or in new _Config class) will catch and log the error.

However, this isn't very noticeable and we should probably fail harder (e.g. like exiting with an error). Other things which write config-files into the node dir should also be examined.

In this particular case, if announcement-seqnum *doesn't* get written (and the user doesn't notice the error in the logs) then the next time we try and make an announcement (next re-start) the introducer will reject it.