naughty: Move the boilerplate rc.lua error handling to naughty.

This removes the imperative "mutex" logic from rc.lua, where it doesn't
belong. It also makes it closer to the "vision" of making `rc.lua` fully
modular.

lib/naughty/core.lua

@@ -512,6 +512,12 @@ function naughty.set_expiration_paused(p)
     end
 end
 
+--- Emitted when an error occurred and requires attention.
+-- @signal request::display_error
+-- @tparam string message The error message.
+-- @tparam boolean startup If the error occurred during the initial loading of
+--  rc.lua (and thus caused the fallback to kick in).
+
 --- Emitted when a notification is created.
 -- @signal added
 -- @tparam naughty.notification notification The notification object

lib/naughty/init.lua

@@ -5,6 +5,7 @@
 ---------------------------------------------------------------------------
 
 local naughty = require("naughty.core")
+local capi = {awesome = awesome}
 if dbus then
     naughty.dbus = require("naughty.dbus")
 end
@@ -17,6 +18,27 @@ naughty.container = require("naughty.container")
 naughty.action = require("naughty.action")
 naughty.notification = require("naughty.notification")
 
+-- Handle runtime errors during startup
+if capi.awesome.startup_errors then
+    naughty.emit_signal(
+        "request::display_error", capi.awesome.startup_errors, true
+    )
+end
+
+-- Handle runtime errors after startup
+do
+    local in_error = false
+    capi.awesome.connect_signal("debug::error", function (err)
+        -- Make sure we don't go into an endless error loop
+        if in_error then return end
+        in_error = true
+
+        naughty.emit_signal("request::display_error", tostring(err), false)
+
+        in_error = false
+    end)
+end
+
 return naughty
 
 -- vim: filetype=lua:expandtab:shiftwidth=4:tabstop=8:softtabstop=4:textwidth=80