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>

terminal.go

@@ -360,6 +360,9 @@ func (s *Server) runTerminal(ws *websocket.Conn, c *orgCtx, siteID string, cols,
 		problem(err.Error())
 		return
 	}
+	// What the shell writes to its error output (including why it could not start) goes to the
+	// terminal like everything else, instead of being thrown away.
+	sess.Stderr = termWriter(func(p []byte) error { return send(termFrame{data: append([]byte(nil), p...)}) })
 	if err := sess.Shell(); err != nil {
 		problem(fmt.Sprintf("Couldn't start a shell: %v", err))
 		return
@@ -566,3 +569,13 @@ func clipText(s string, n int) string {
 	}
 	return s
 }
+
+// termWriter sends what is written to it to the browser as terminal output.
+type termWriter func(p []byte) error
+
+func (w termWriter) Write(p []byte) (int, error) {
+	if err := w(p); err != nil {
+		return 0, err
+	}
+	return len(p), nil
+}