mwscript.py: Make script argument required

Make the script argument required for scap mwscript and scap php.

Before:

$ scap php --directory /srv/mediawiki-staging --user www-data  
Could not open input file: None  

After:

$ scap php --directory /srv/mediawiki-staging --user www-data  
usage: scap php [-h] [--conf CONF_FILE] [--no-shared-authsock] [-D <name>:<value>] [--verbose] [--environment ENVIRONMENT] [--no-log-message] [--no-local-config] --directory  
                DIRECTORY --user USER [--network]  
                script ...  
scap php: error: the following arguments are required: script, arguments  

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Related MRs:

• !871 (merged)
• !870 (merged) (This MR)
• master