Add link to Github project in Error message about cli

[LukasFritzeDev]

Just recognised some errors already include this link:

╭──────────────────────────────────────────────────────────────────────────────╮
│ ERROR                                                                        │
│                                                                              │
│ An error occurred while executing this command:                              │
│                                                                              │
│   Error: Filter flag has an invalid value                                    │
│                                                                              │
│ If you believe this to be a bug, please open an issue at                     │
│ https://github.com/mittwald/cli/issues/new                                   │
│ (​https://github.com/mittwald/cli/issues/new​).                              │
╰──────────────────────────────────────────────────────────────────────────────╯

But the formatting could be improved (there is no need to print the link twice) and some other errors (see #54) are missing the link.

[martin-helmich]

Regarding the duplicate link: Is this some rendering variation of the <Link> component? AFAIK it should just render a clickable link into the terminal -- maybe this is some kind of fallback behaviour in case the terminal emulator doesn't support links?