Running binary with only the argument tla causes panic instead of giving warning to user.

[danwt]

Bug:

Running binary ./modelator tla panics.

Expected:

./modelator --help gives

SUBCOMMANDS:
    apalache    Generate TLA+ traces using Apalache
    tla         Generate TLA+ test cases and parse TLA+ traces
    tlc         Generate TLA+ traces using TLC

so it is expected that a user will try to pass only tla arg.

Log:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', modelator/src/cli/mod.rs:29:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Additional info:

Tested on release in versions 0.2.0 and (06ed539eb) latest

[andrey-kuprianov]

Fixed by e4a94f9; closing