fix(deps): raise exception when pandas is installed but db-dtypes is not

[tswast]

db-dtypes is already present in the pandas "extras", but this PR ensures that if pandas is present and db-dtypes is not, a more understandable error message is raised.

google/cloud/bigquery/_pandas_helpers.py:991: ValueError
____________________________________ test_list_rows_nullable_scalars_extreme_dtypes[10] _____________________________________

    # Copyright 2019 Google LLC
    #
    # Licensed under the Apache License, Version 2.0 (the "License");
    # you may not use this file except in compliance with the License.
    # You may obtain a copy of the License at
    #
    #     http://www.apache.org/licenses/LICENSE-2.0
    #
    # Unless required by applicable law or agreed to in writing, software
    # distributed under the License is distributed on an "AS IS" BASIS,
    # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    # See the License for the specific language governing permissions and
    # limitations under the License.
    
    """Shared helper functions for connecting BigQuery and pandas."""
    
    import concurrent.futures
    from datetime import datetime
    import functools
    from itertools import islice
    import logging
    import queue
    import warnings
    
    try:
        import pandas  # type: ignore
    
        pandas_import_exception = None
    except ImportError as exc:  # pragma: NO COVER
        pandas = None
        pandas_import_exception = exc
    else:
        import numpy
    
    try:
>       import db_dtypes  # type: ignore
E       ModuleNotFoundError: No module named 'db_dtypes'

google/cloud/bigquery/_pandas_helpers.py:36: ModuleNotFoundError

The above exception was the direct cause of the following exception:

bigquery_client = <google.cloud.bigquery.client.Client object at 0x11e2d3580>
scalars_extreme_table = 'swast-scratch.python_bigquery_tests_system_20220330160830_ffff89.scalars_extreme_jsonl0x3ffeb'
max_results = 10

    @pytest.mark.parametrize(
        ("max_results",),
        (
            (None,),
            (10,),
        ),  # Use BQ Storage API.  # Use REST API.
    )
    def test_list_rows_nullable_scalars_extreme_dtypes(
        bigquery_client, scalars_extreme_table, max_results
    ):
        # TODO(GH#836): Avoid INTERVAL columns until they are supported by the
        # BigQuery Storage API and pyarrow.
        schema = [
            bigquery.SchemaField("bool_col", enums.SqlTypeNames.BOOLEAN),
            bigquery.SchemaField("bignumeric_col", enums.SqlTypeNames.BIGNUMERIC),
            bigquery.SchemaField("bytes_col", enums.SqlTypeNames.BYTES),
            bigquery.SchemaField("date_col", enums.SqlTypeNames.DATE),
            bigquery.SchemaField("datetime_col", enums.SqlTypeNames.DATETIME),
            bigquery.SchemaField("float64_col", enums.SqlTypeNames.FLOAT64),
            bigquery.SchemaField("geography_col", enums.SqlTypeNames.GEOGRAPHY),
            bigquery.SchemaField("int64_col", enums.SqlTypeNames.INT64),
            bigquery.SchemaField("numeric_col", enums.SqlTypeNames.NUMERIC),
            bigquery.SchemaField("string_col", enums.SqlTypeNames.STRING),
            bigquery.SchemaField("time_col", enums.SqlTypeNames.TIME),
            bigquery.SchemaField("timestamp_col", enums.SqlTypeNames.TIMESTAMP),
        ]
    
        df = bigquery_client.list_rows(
            scalars_extreme_table,
            max_results=max_results,
            selected_fields=schema,
>       ).to_dataframe()

tests/system/test_pandas.py:1084: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
google/cloud/bigquery/table.py:1925: in to_dataframe
    _pandas_helpers.verify_pandas_imports()
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    def verify_pandas_imports():
        if pandas is None:
            raise ValueError(_NO_PANDAS_ERROR) from pandas_import_exception
        if db_dtypes is None:
>           raise ValueError(_NO_DB_TYPES_ERROR) from db_dtypes_import_exception
E           ValueError: Please install the 'db-dtypes' package to use this function.

google/cloud/bigquery/_pandas_helpers.py:991: ValueError

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• Appropriate docs were updated (if necessary)

Fixes #1188 🦕