sl0ppty: say what the config could not honour

A config that half-applied said nothing. An unknown shader, a binding
that does not parse, an include that is not there: the file loaded, that
line did not, and the session announced "config reloaded" as though
nothing had happened. Every one of those messages was already being
produced -- config_load has filled `err` on a successful load since it
was written -- and then dropped on the floor by every caller, because a
non-zero return meant "fine".

So the complaint from the last load is kept, and the front end says it
where it would have said "config reloaded". Once at startup too: a config
that loaded with a complaint still started the session, and the stderr
line goes where nobody attached can read it. The `reload` control verb
answers `{"ok":true,"warning":"..."}` -- ok, because the file did apply,
with the reason attached for a script that cares.

Two things fell out of writing the test for it.

An include's complaint was reported by nobody when the file in the middle
was fine. apply_includes only looked at the return value, so a theme that
loaded while *its* include was missing swallowed the message: the child
returned true, and true meant there was nothing to say. The complaint now
travels up whether or not the include itself loaded.

And a complaint now carries the name of the file it happened in, exactly
once -- added by the file that produced it, never by the files that
included it, because prefixing at every level builds a chain that
truncates before the part that matters. So:

    t.kdl: unknown shader: bloom
    keys.kdl: line 2: unknown action fly
    b.kdl: includes nested too deep (a cycle?)
    cannot open /home/you/.config/sl0ppty/themes/nope.kdl

which is enough to know which file to open, in a message that fits in a
toast.

1 files changed, 5 insertions, 2 deletions

diff --git a/config/config.kdl b/config/config.kdl
index 7e242c9..1c2bd88 100644
--- a/config/config.kdl
+++ b/config/config.kdl
@@ -62,8 +62,11 @@
 // or `states` block replaces the one it inherited, like every one of those.
 //
 // A file that will not load is skipped and the rest of the config still
-// applies. Losing your keybindings over a mistyped theme name would be the
-// worse answer.
+// applies -- losing your keybindings over a mistyped theme name would be the
+// worse answer -- and the session says which file it could not honour, in the
+// same place it says "config reloaded". That goes for anything a config
+// complains about: an unknown shader, a binding that does not parse, an include
+// that is not there. The file applied; one line of it did not; you get told.
 
 // ── geometry ────────────────────────────────────────────────────────────────