[Feature request] Validate arguments before scrapping

[ngosang]

We should validate the following parameters at the start. I'm getting validation errors after waiting for scrapping. I have to correct the arguments and start again to see if the import works.

• source is valid
• destination is valid
• file exists and is json

$ python3 transfer_ratings.py --s trakt -d letterboxed
===== Trakt: performing login
===== loaded 2 movies from /home/xxxx/RatS/RatS/exports/20210326181821_Trakt_mod.json

ERROR: No inserter matching 'LETTERBOXED' found.

===== ABORTING =====
Available inserters:
 - ALLOCINE 
 - CRITICKER 
 - FILMAFFINITY 
 - FILMTIPSET 
 - ICHECKMOVIES 
 - IMDB 
 - LETTERBOXD 
 - LISTAL 
 - METACRITIC 
 - MOVIELENS 
 - MOVIEPILOT 
 - PLEX 
 - ROTTENTOMATOES 
 - TMDB 
 - TRAKT 

[StegSchreck]

Your suggestion makes sense. Thanks for your input!