Terminal: show the shell's error output, so a missing shell is explained

The shell's stderr was discarded, so a shell that could not start looked like an
immediate disconnect. It now goes to the terminal like other output, followed by
the exit code. An end-to-end test covers a machine with no shell. Builds against
Bamboo v0.1.10, which says why when there is no shell to run.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

e2e/terminal_relay_test.go

@@ -137,6 +137,33 @@ func TestWebTerminalWithoutAnyOpenPort(t *testing.T) {
 		t.Errorf("the test opened a port after all: %+v", maps.Mappings)
 	}
 
+	// a machine with no shell (the published container image has none) says why, instead of the
+	// terminal just disconnecting: the shell's error output reaches the browser, then the exit code
+	if sshd.Supported {
+		t.Setenv("SHELL", "/nonexistent/shell")
+		ws := dial()
+		defer ws.Close()
+		var out bytes.Buffer
+		var exit termMsg
+		for exit.Type != "exit" {
+			ws.SetReadDeadline(time.Now().Add(15 * time.Second))
+			var f []byte
+			if err := websocket.Message.Receive(ws, &f); err != nil {
+				t.Fatalf("waiting for the shell to end (got %q): %v", out.String(), err)
+			}
+			if len(f) > 0 && f[0] == '{' {
+				if m := parseTermMsg(f); m.Type == "exit" {
+					exit = m
+				}
+				continue
+			}
+			out.Write(f)
+		}
+		if exit.Code != 127 || !strings.Contains(out.String(), "no shell to run") {
+			t.Errorf("a missing shell: exit %d, output %q", exit.Code, out.String())
+		}
+	}
+
 	// the SSH server still decides who gets in
 	other := newSSHSigner(t)
 	if err := os.WriteFile(keys, ssh.MarshalAuthorizedKey(other.PublicKey()), 0o600); err != nil {