Improved errors

[phoenixr-codes]

The current implementation used normal Python exceptions. For end users these errors might be confusing – especially those unfamiliar with Python.

Here are some ideas:

> prog foo bar baz
               ~~~
error: could not convert 'baz' into `int`

> prog foo bar
               ~~~
error: missing required positional AMOUNT

> prog -f -g -j
       ~~    ~~
error: flag 'f' and 'j' cannot be used together