changed warning to error if the directory does not have write permission

[LuitelSamikshya]

This PR displays an error if the user attempts to store response in a directory with no write permission.
Test: Create a directory without write permission, run echo 'example.com' | ./httpx -store-response-dir ./no_write_permission_dir/

[Mzack9999]

$ mkdir dir_no_write
$ chmod 400 dir_no_write/
$ touch dir_no_write/test.txt
touch: dir_no_write/test.txt: Permission denied
$ echo 'example.com' | go run . -store-response-dir ./dir_no_write/

    __    __  __       _  __
   / /_  / /_/ /_____ | |/ /
  / __ \/ __/ __/ __ \|   /
 / / / / /_/ /_/ /_/ /   |
/_/ /_/\__/\__/ .___/_/|_|
             /_/              v1.2.1

                projectdiscovery.io

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Developers assume no liability and are not responsible for any misuse or damage.
[ERR] Could not write response at path 'dir_no_write/example.com.txt', to disk: open dir_no_write/example.com.txt: permission denied
https://example.com