Show a nicer message on invalid or no module

[Glutexo]

$ ./onigumo
** (FunctionClauseError) no function clause matching in Onigumo.CLI.main/1    
    
    The following arguments were given to Onigumo.CLI.main/1:
    
        # 1
        []
    
    (onigumo 0.1.0) lib/cli.ex:2: Onigumo.CLI.main/1
    (elixir 1.14.3) lib/kernel/cli.ex:131: anonymous fn/3 in Kernel.CLI.exec_fun/2 

[Glutexo]

Now, with OptionParser in place, the error looks different:

$ onigumo
** (MatchError) no match of right hand side value: :error
    (onigumo 0.1.0) lib/cli.ex:16: Onigumo.CLI.main/1
    (elixir 1.17.2) lib/kernel/cli.ex:136: anonymous fn/3 in Kernel.CLI.exec_fun/2

[Glutexo]

Fixed by #249.