(improvement, fern): Add better error for invalid generators.yml

[amckinney]

This improves the fern compiler's error reporting when the user's generators.yml is invalid. For example, suppose you have the following configuration with duplicate map keys (i.e. publish):

groups:
  publish:
    generators:
      - name: fernapi/fern-go-sdk
        version: 0.21.1
        github:
          repository: acme/acme-go
  publish:
    generators:
      - name: fernapi/fern-python-sdk
        version: 1.4.0
        github:
          repository: acme/acme-python

The CLI would previously fail with an unhelpful error message like the following:

$ fern generate
Missing file: api.yml

This was because we would ignore any errors thrown by the generators.yml, even if the file existed. This refactors the approach so that we actually act upon the errors thrown by the generators.yml parser and surface it to the user. Now, the user will see the following:

$ fern generate
Failed to parse generators.yml: duplicated mapping key

[dsinghvi]

Thank you! 🚀 If there are multiple generators.yml files should we be logging the filepath of the erroneous one?

[amckinney]

Great call, updated!