amazon-price-alert doesn't validate frequency entry

[habereet]

Describe the bug In amazon-price-alert when entering the rate that the URL should be checked, entering a value that isn't one of the options doesn't return an error.

To Reproduce Steps to reproduce the behavior:

1. Install the required libraries
2. Run the script - python amazon_scraper.py
3. Enter a valid URL for the URL when requested for the link to the product
4. Enter an email address or a string that looks like an email address
5. When asked for a frequency, enter a value that's not in the list
   • number - The script continues as if the user entered an accepted value
   • character - the script errors with a ValueError
   • nothing - the script errors with a ValueError

Expected behavior The script should handle exceptions and test input. When not valid, it should prompt the user for a valid value then exit if they're unable to provide one.

Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: Ubuntu 18.04 (on WSL2)
• Python: 3.8.6

[habereet]

Please assign to me

[habereet]

Submiitted Pull Request #362 to resolve this.