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**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`): n/a

**Environment**: 

- **Cloud provider or hardware configuration**: n/a
- **OS** (e.g. from /etc/os-release): n/a
- **Kernel** (e.g. `uname -a`): n/a
- **Install tools**: n/a
- **Others**: n/a

**What happened**:
A sphinx warning unrelated to spelling issues running `sphinx-build` resulted in an instance of `SpellingError` to cause a docs build failure.

```
SpellingError(
    file_path=None,
    line_no=None,
    spelling=None,
    suggestion=None,
    context_line=None,
    message=(
        f"Sphinx spellcheck returned non-zero exit status: {completed_proc.returncode}."
    )
)

# sphinx.errors.SphinxWarning: /opt/airflow/docs/apache-airflow-providers-google/_api/drive/index.rst:document isn't included in any toctree
```
The actual issue was that I failed to include an `__init__.py` file in a directory that I created.

<!-- (please include exact error messages if you can) -->

**What you expected to happen**:
I think an exception should be raised unrelated to a spelling error. Preferably one that would indicate that there's a directory that's missing an init file, but at least a generic error unrelated to spelling
<!-- What do you think went wrong? -->

**How to reproduce it**:
Create a new plugin directory (e.g. `airflow/providers/google/suite/sensors`) and don't include an `__init__.py` file, and run `./breeze build-docs -- --docs-only -v`
<!---

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--->


**Anything else we need to know**:
I'm specifically referring to lines 139 to 150 in `docs/exts/docs_build/docs_builder.py`
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docs/exts/docs_build/spelling_checks.py
#
#
import os
import re
from functools import total_ordering
from typing import Dict, List, NamedTuple, Optional
from airflow.utils.code_utils import prepare_code_snippet
CURRENT_DIR = os.path.abspath(os.path.join(os.path.dirname(__file__)))
DOCS_DIR = os.path.abspath(os.path.join(CURRENT_DIR, os.pardir, os.pardir))
@total_ordering
class SpellingError(NamedTuple):