[MINOR][SS] Fix a missing space in UnsupportedOperationChecker error message

## What changes were proposed in this pull request?

Also went through the same file to ensure other string concatenation are correct.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #17691 from zsxwing/fix-error-message.

(cherry picked from commit 39e303a8)
Signed-off-by: Shixiong Zhu <shixiong@databricks.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/analysis/UnsupportedOperationChecker.scala

@@ -166,7 +166,7 @@ object UnsupportedOperationChecker {
           throwError("Limits are not supported on streaming DataFrames/Datasets")
 
         case Sort(_, _, _) if !containsCompleteData(subPlan) =>
-          throwError("Sorting is not supported on streaming DataFrames/Datasets, unless it is on" +
+          throwError("Sorting is not supported on streaming DataFrames/Datasets, unless it is on " +
             "aggregated DataFrame/Dataset in Complete output mode")
 
         case Sample(_, _, _, _, child) if child.isStreaming =>