[SPARK-30231][SQL][PYTHON][FOLLOWUP] Make error messages clear in PySpark df.explain

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

This pr is a followup of #26861 to address minor comments from viirya.

### Why are the changes needed?

For better error messages.

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

No.

### How was this patch tested?

Manually tested.

Closes #26886 from maropu/SPARK-30231-FOLLOWUP.

Authored-by: Takeshi Yamamuro <yamamuro@apache.org>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

python/pyspark/sql/dataframe.py

@@ -305,11 +305,12 @@ class DataFrame(object):
         is_mode_case = mode is not None and isinstance(mode, basestring)
 
         if not is_no_argument and not (is_extended_case or is_mode_case):
-            argtypes = [
-                str(type(arg)) for arg in [extended, mode] if arg is not None]
-            raise TypeError(
-                "extended (optional) and mode (optional) should be a bool and str; "
-                "however, got [%s]." % ", ".join(argtypes))
+            if extended is not None:
+                errMsg = "extended (optional) should be provided as bool" \
+                    ", got {0}".format(type(extended))
+            else:  # For mode case
+                errMsg = "mode (optional) should be provided as str, got {0}".format(type(mode))
+            raise TypeError(errMsg)
 
         # Sets an explain mode depending on a given argument
         if is_no_argument:

sql/core/src/main/scala/org/apache/spark/sql/Dataset.scala

@@ -547,6 +547,7 @@ class Dataset[T] private[sql](
     }
   }
 
+  // This method intends to be called from PySpark DataFrame
   private[sql] def toExplainString(mode: String): String = {
     mode.toLowerCase(Locale.ROOT) match {
       case "simple" => toExplainString(ExplainMode.Simple)