Fail verbosely

Fail verbosely

Along the way, we also fixed the handling of the `*_padding`,
`window_gap` and `border_width` settings. The previous behavior was the
result of a bad decision (9fed780), as a response to #141. Many issues
followed: #143, #158, #260, etc. We now handle those settings as
intuitively as possible.

We also fixed a potential segfault in `cmd_node`, triggered by `bspc
node -d`[sic].

Fixes #402 #252.