[SPARK-45783][PYTHON][CONNECT] Improve error messages when Spark Connect mode is enabled but remote URL is not set

### What changes were proposed in this pull request?

This PR improves the error messages when `SPARK_CONNECT_MODE_ENABLED` is defined but neither `spark.remote` option nor the `SPARK_REMOTE` env var is set.

### Why are the changes needed?

To improve the error message. Currently the error looks like a bug:
```
url = opts.get("spark.remote", os.environ.get("SPARK_REMOTE"))

>          if url.startswith("local"):
E          AttributeError: 'NoneType' object has no attribute 'startswith'
```

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

New unit test

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #43653 from allisonwang-db/spark-45783-fix-url-err.

Authored-by: allisonwang-db <[email protected]>
Signed-off-by: Hyukjin Kwon <[email protected]>

Author: allisonwang-db

python/pyspark/sql/session.py

@@ -481,6 +481,14 @@ def getOrCreate(self) -> "SparkSession":
                         ):
                             url = opts.get("spark.remote", os.environ.get("SPARK_REMOTE"))
 
+                            if url is None:
+                                raise RuntimeError(
+                                    "Cannot create a Spark Connect session because the "
+                                    "Spark Connect remote URL has not been set. Please define "
+                                    "the remote URL by setting either the 'spark.remote' option "
+                                    "or the 'SPARK_REMOTE' environment variable."
+                                )
+
                             if url.startswith("local"):
                                 os.environ["SPARK_LOCAL_REMOTE"] = "1"
                                 RemoteSparkSession._start_connect_server(url, opts)

python/pyspark/sql/tests/test_session.py

@@ -17,6 +17,7 @@
 
 import os
 import unittest
+import unittest.mock
 
 from pyspark import SparkConf, SparkContext
 from pyspark.sql import SparkSession, SQLContext, Row
@@ -187,6 +188,11 @@ def test_active_session_with_None_and_not_None_context(self):
             if sc is not None:
                 sc.stop()
 
+    def test_session_with_spark_connect_mode_enabled(self):
+        with unittest.mock.patch.dict(os.environ, {"SPARK_CONNECT_MODE_ENABLED": "1"}):
+            with self.assertRaisesRegex(RuntimeError, "Cannot create a Spark Connect session"):
+                SparkSession.builder.appName("test").getOrCreate()
+
 
 class SparkSessionTests4(ReusedSQLTestCase):
     def test_get_active_session_after_create_dataframe(self):