Prevent a possible assert in the debugger when a thread detaches (#34955)

Prevent a possible assert in the debugger when a thread detaches (#34955)

When a new thread is attached to the VM, the debugger will add it to the
`thread_to_tls` hash table. When that thread detaches, it will be
removed. Once the thread is attached, if a client is debugging, the
client can use the `CMD_THREAD_GET_FRAME_INFO` command to ask for
details about that thread.

This is a possibility that the thread detaches before the
`CMD_THREAD_GET_FRAME_INFO` command is processed, so the thread may not
exist the the `thread_to_tls` hash table any more.

This is a race condition, but is a valid state, so instead of asserting,
the debugger agent should indicate to the client that the thread no
longer exists, using `ERR_UNLOADED`.