Fixes #2220

• Initialization errors
• Login errors
• Non-fatal error while downloading
• --abort-on
• SIGINT / Control-C
• Documentation

Here's the general idea of how each situation is handled:

• Non fatal error (1): Before exiting, it is checked if something was logged to be repeated at the end (those are the real warning/erros that require attention). If so, the exit code is 1.
• Initialization failure (2): returned by argparse for invalid options and other parsing errors; other errors that instaloader itself detects raise InvalidArgumentException, which is caught at main().
  • InvalidArgumentException is also used for other errors, that generally can only be triggered when used as a module. I don't think that's a problem, but another exception could be used specifically for those initialization errors.
• Login failure (3): raises the new LoginException. Some more specific login exceptions were made into subclasses.
• Download aborted (4) and user aborted (5): these were already caught at _main, it just sets a flag with the status to be returned.

Some notes:

• I created a new exit code 99 for any uncaught InstaloaderException (at the very end of __main__.py). But maybe that except is not necessary: hopefully no exception should reach there, and if it does, maybe letting the program crash (and include the stack trace) is more appropriate.
• I've made the failure when no IG cookies are found an actual failure. It used to just print a message and carry on (anonymously)
• When nothing is done (no targets are specified), I'm returning non-fatal error, but initialization failure could also be appropriate, especially when the user was not logged in. But having two different status codes for two very similar situations is probably too confusing, so I used non-fatal error for both cases.