Fix string formatting in exception constructors across codebase

[bysiber]

Several raise statements across the codebase pass format arguments to exception constructors using a comma instead of % operator:

# Before (comma creates a tuple message):
raise ValueError("message %s", value)

# After (proper string formatting):
raise ValueError("message %s" % value)

When Python's Exception.__init__ receives multiple arguments, str(e) shows the raw tuple representation instead of a formatted message. For example, ValueError("unsupported auth_mode %s", "digest") displays as:

ValueError: ('unsupported auth_mode %s', 'digest')

instead of:

ValueError: unsupported auth_mode digest

Affected locations:

• web.py: _convert_header_value, xsrf_token, stream_request_body, _has_stream_request_body
• websocket.py: _PerMessageDeflateCompressor.__init__, _PerMessageDeflateDecompressor.__init__, _process_server_headers
• simple_httpclient.py: HTTP basic auth mode validation
• netutil.py: Unix socket bind validation
• concurrent.py: run_on_executor argument validation