feat(watch): add on_error callback

Problem: Watcher failures are reported inconsistently, and callers
cannot detect when a watcher is no longer usable.

Solution: Route startup and event failures and unexpected inotifywait
exits through an optional on_error callback. Log to nvim-watch.log by
default. Ignore disappearing child directories and exits caused by
cancellation. Let LSP log failures and notify once per message, using
INFO for missing roots and ERROR otherwise, without interrupting
registration.

AI-assisted

runtime/lua/vim/lsp/_watchfiles.lua

@@ -2,6 +2,7 @@ local bit = require('bit')
 local glob = vim.glob
 local watch = vim._watch
 local log = require('vim.lsp.log')
+local notify = require('vim._core.util').notify
 local protocol = require('vim.lsp.protocol')
 local lpeg = vim.lpeg
 
@@ -185,6 +186,20 @@ function M.register(reg, client_id)
         -- match a *particular* pattern+kind pair.
         include_pattern = include_pattern,
         exclude_pattern = M._poll_exclude_pattern,
+        on_error = function(err)
+          local name = string.format('LSP[%s]', client.name)
+          local message = string.format('file watcher failed for %s', base_dir)
+          local level = vim.log.levels.ERROR
+          -- Servers may register a nonexistent baseUri. Keep this informational
+          -- and continue registering the other watchers.
+          if err:match('^ENOENT:') then
+            level = vim.log.levels.INFO
+            log.info(name, message, err)
+          else
+            log.error(name, message, err)
+          end
+          notify(name, message .. ': ' .. err, level, true)
+        end,
       }, callback(base_dir))
     )
   end