[lldb] Fixed an invalid error message in the DAP disconnect response (#92345)

The `disconnect` response contains the `error` message with invalid
characters (a junk data). To reproduce this issue it is enough to run
the `TestDAP_commands` test on Windows host and Linux target. The test
will fail to run ELF file on Windows and dap_server will be disconnected
unexpectedly.

Note dap_server hangs if read_packet() cannot decode JSON with invalid
characters. read_packet() must return None in this case instead of an
exception. But dap_server does not require any fix after this patch.