fix(tui): wait for embedded server's exit code

Uses the undocumented "error_exit" UI event for a different purpose:
When :detach is used on the server, send an "error_exit" with 0 `status`
to indicate that the server shouldn't wait for client exit.

test/functional/terminal/tui_spec.lua

@@ -33,6 +33,28 @@ local assert_log = t.assert_log
 
 local testlog = 'Xtest-tui-log'
 
+describe('TUI', function()
+  it('exit status 1 and error message with server --listen error #34365', function()
+    clear()
+    local addr_in_use = api.nvim_get_vvar('servername')
+    local screen = tt.setup_child_nvim(
+      { '--listen', addr_in_use, '-u', 'NONE', '-i', 'NONE' },
+      { extra_rows = 10, cols = 60 }
+    )
+    -- When the address is very long, the error message may be only partly visible.
+    if #addr_in_use <= 600 then
+      screen:expect({
+        any = vim.pesc(
+          ('%s: Failed to --listen: address already in use:'):format(
+            is_os('win') and 'nvim.exe' or 'nvim'
+          )
+        ),
+      })
+    end
+    screen:expect({ any = vim.pesc('[Process exited 1]') })
+  end)
+end)
+
 describe('TUI :detach', function()
   it('does not stop server', function()
     local job_opts = { env = {} }
@@ -2227,15 +2249,20 @@ describe('TUI', function()
 
   it('no assert failure on deadly signal #21896', function()
     exec_lua([[vim.uv.kill(vim.fn.jobpid(vim.bo.channel), 'sigterm')]])
-    screen:expect {
-      grid = [[
+    screen:expect([[
       Nvim: Caught deadly signal 'SIGTERM'              |
                                                         |
       [Process exited 1]^                                |
                                                         |*3
       {3:-- TERMINAL --}                                    |
-    ]],
-    }
+    ]])
+  end)
+
+  it('exit status 1 and error message with deadly signal sent to server', function()
+    local _, server_pid = child_session:request('nvim_call_function', 'getpid', {})
+    exec_lua([[vim.uv.kill(..., 'sigterm')]], server_pid)
+    screen:expect({ any = vim.pesc([[Nvim: Caught deadly signal 'SIGTERM']]) })
+    screen:expect({ any = vim.pesc('[Process exited 1]') })
   end)
 
   it('no stack-use-after-scope with cursor color #22432', function()