Update ivar name in send_fail_response

[st0012]

There's no @web_sock anymore. So when sending error response it'll cause:

❯ exe/rdbg --open=chrome target.rb
DEBUGGER: Debugger can attach via TCP/IP (127.0.0.1:49402)
DEBUGGER: wait for debugger connection...
DEBUGGER: Connected.
#<Thread:0x00007f8bfd23a4c0@DEBUGGER__::SESSION@server /Users/st0012/projects/debug/lib/debug/session.rb:148 run> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
        6: from /Users/st0012/projects/debug/lib/debug/session.rb:168:in `block in activate'
        5: from /Users/st0012/projects/debug/lib/debug/session.rb:198:in `session_server_main'
        4: from /Users/st0012/projects/debug/lib/debug/session.rb:295:in `process_event'
        3: from /Users/st0012/projects/debug/lib/debug/server_cdp.rb:590:in `cdp_event'
        2: from /Users/st0012/projects/debug/lib/debug/server_cdp.rb:509:in `fail_response'
        1: from /Users/st0012/projects/debug/lib/debug/server_cdp.rb:486:in `respond_fail'
/Users/st0012/projects/debug/lib/debug/server_cdp.rb:247:in `send_fail_response': {:id=>63, :error=>"Error: Can not evaluate: \\"bar\\""} is not a symbol nor a string (TypeError)

The error is confusing as it essentially calls nil.send, so it expects the argument to be a symbol.

[st0012]

@ono-max can you give this a look? thx

[ono-max]

Good catch.
Thank you!