After having created a lock file 2.0, running `poetry lock` with poetry
1.2.1 results in the following output:

```
The lock file is not compatible with the current version of Poetry.
Upgrade Poetry to be able to read the lock file or, alternatively, regenerate the lock file with the `poetry lock` command.
```

Ironically, the error message proposes to run `poetry lock` which
results in this error message.

Further, it doesn't make sense that `poetry lock` fails because it
creates a new lock file from scratch (in contrast to `poetry lock
--no-update`).

Running `poetry lock` is now also possible if there is a broken lock file.

Resolves: #1196