Improve the credential check. (#183)

### Description

Improves the credential check.

It will show a consistent error message when missing the required fields, `host`, `http_path`, and `token`.

For example:

```
The config 'host' is required to connect to Databricks
```

dbt/adapters/databricks/connections.py

@@ -16,6 +16,7 @@
     Optional,
     Sequence,
     Tuple,
+    cast,
 )
 
 from agate import Table
@@ -57,14 +58,15 @@
 
 @dataclass
 class DatabricksCredentials(Credentials):
-    host: str
     database: Optional[str]  # type: ignore[assignment]
+    host: Optional[str] = None
     http_path: Optional[str] = None
     token: Optional[str] = None
-    connect_retries: int = 0
-    connect_timeout: int = 10
     session_properties: Optional[Dict[str, Any]] = None
     connection_parameters: Optional[Dict[str, Any]] = None
+
+    connect_retries: int = 0
+    connect_timeout: int = 10
     retry_all: bool = False
 
     _ALIASES = {
@@ -131,7 +133,7 @@ def type(self) -> str:
 
     @property
     def unique_field(self) -> str:
-        return self.host
+        return cast(str, self.host)
 
     def connection_info(self, *, with_aliases: bool = False) -> Iterable[Tuple[str, Any]]:
         as_dict = self.to_dict(omit_none=False)
@@ -403,7 +405,7 @@ def list_schemas(self, database: str, schema: Optional[str] = None) -> Table:
     @classmethod
     def validate_creds(cls, creds: DatabricksCredentials, required: List[str]) -> None:
         for key in required:
-            if not hasattr(creds, key):
+            if not getattr(creds, key):
                 raise dbt.exceptions.DbtProfileError(
                     "The config '{}' is required to connect to Databricks".format(key)
                 )
@@ -450,7 +452,7 @@ def open(cls, connection: Connection) -> Connection:
             return connection
 
         creds: DatabricksCredentials = connection.credentials
-        exc: Optional[Exception] = None
+        cls.validate_creds(creds, ["host", "http_path", "token"])
 
         user_agent_entry = f"dbt-databricks/{__version__}"
 
@@ -459,20 +461,14 @@ def open(cls, connection: Connection) -> Connection:
             cls.validate_invocation_env(invocation_env)
             user_agent_entry = f"{user_agent_entry}; {invocation_env}"
 
-        if creds.http_path is None:
-            raise dbt.exceptions.DbtProfileError(
-                "`http_path` must set when" " using the dbsql method to connect to Databricks"
-            )
-        required_fields = ["host", "http_path", "token"]
-
-        cls.validate_creds(creds, required_fields)
-
         connection_parameters = creds.connection_parameters.copy()  # type: ignore[union-attr]
 
         http_headers: List[Tuple[str, str]] = cls.get_all_http_headers(
             connection_parameters.pop("http_headers", {})
         )
 
+        exc: Optional[Exception] = None
+
         for i in range(1 + creds.connect_retries):
             try:
                 # TODO: what is the error when a user specifies a catalog they don't have access to

dbt/adapters/databricks/impl.py

@@ -288,7 +288,9 @@ def submit_python_job(
         command_name += "-" + str(uuid.uuid1())
 
         api_client = Api12Client(
-            host=credentials.host, token=cast(str, credentials.token), command_name=command_name
+            host=cast(str, credentials.host),
+            token=cast(str, credentials.token),
+            command_name=command_name,
         )
 
         try: