Fix misleading warning when evaluating Jinja in dbt_project.yml fails

[fornwall]

Fixes #387.

For a dbt_project.yml with the e.g. following model section:

models:
  my_model:
    +hours_to_expiration: "{{ env_var('MY_ENV_VAR') }}"

The behaviour before this PR when the MY_ENV_VAR variable is not defined:

warning: (will error post beta) dbt1060: Ignored unexpected key "my_model". YAML path: my_model.
[..]
warning: dbt1000: Warnings marked (will error post beta) will turn into errors before leaving beta. Please fix them.
suggestion: Try the autofix script: https://github.com/dbt-labs/dbt-autofix

The behaviour after this PR when the MY_ENV_VAR variable is not defined:

error: dbt1013: Failed to parse my_model: Failed to eval the compiled Jinja expression invalid operation: 'env_var': environment variable 'MY_ENV_VAR' not found

See #385 and #386 for two minor polish changes for this error message.

It would be even better to pinpoint +hours_to_expiration in the error message, but that seems to require more invasive changes to the yaml parsing so is left for the future now.

[fornwall]

AFAIK we don't need show_warning_soon_to_be_error here, as the real Ignored unexpected key message for unexpected yaml keys is generated here: https://github.com/dbt-labs/dbt-fusion/blob/main/crates/dbt-jinja-utils/src/serde.rs#L279C14-L279C36, and then dispatched with show_warning_soon_to_be_error here: https://github.com/dbt-labs/dbt-fusion/blob/main/crates/dbt-jinja-utils/src/serde.rs#L253-L263

[fornwall]

I don't know what the best way to add a test for this. Perhaps it's best to use some high level integration test for parsing projects and assert results, to exercise the interactions between the yaml parser and later processing such as error handling?