[SPARK-18259][SQL] Do not capture Throwable in QueryExecution

## What changes were proposed in this pull request?
`QueryExecution.toString` currently captures `java.lang.Throwable`s; this is far from a best practice and can lead to confusing situation or invalid application states. This PR fixes this by only capturing `AnalysisException`s.

## How was this patch tested?
Added a `QueryExecutionSuite`.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #15760 from hvanhovell/SPARK-18259.

sql/core/src/main/scala/org/apache/spark/sql/execution/QueryExecution.scala

@@ -104,7 +104,7 @@ class QueryExecution(val sparkSession: SparkSession, val logical: LogicalPlan) {
     ReuseSubquery(sparkSession.sessionState.conf))
 
   protected def stringOrError[A](f: => A): String =
-    try f.toString catch { case e: Throwable => e.toString }
+    try f.toString catch { case e: AnalysisException => e.toString }
 
 
   /**

sql/core/src/test/scala/org/apache/spark/sql/execution/QueryExecutionSuite.scala

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You 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.
+ */
+package org.apache.spark.sql.execution
+
+import org.apache.spark.sql.AnalysisException
+import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, OneRowRelation}
+import org.apache.spark.sql.test.SharedSQLContext
+
+class QueryExecutionSuite extends SharedSQLContext {
+  test("toString() exception/error handling") {
+    val badRule = new SparkStrategy {
+      var mode: String = ""
+      override def apply(plan: LogicalPlan): Seq[SparkPlan] = mode.toLowerCase match {
+        case "exception" => throw new AnalysisException(mode)
+        case "error" => throw new Error(mode)
+        case _ => Nil
+      }
+    }
+    spark.experimental.extraStrategies = badRule :: Nil
+
+    def qe: QueryExecution = new QueryExecution(spark, OneRowRelation)
+
+    // Nothing!
+    badRule.mode = ""
+    assert(qe.toString.contains("OneRowRelation"))
+
+    // Throw an AnalysisException - this should be captured.
+    badRule.mode = "exception"
+    assert(qe.toString.contains("org.apache.spark.sql.AnalysisException"))
+
+    // Throw an Error - this should not be captured.
+    badRule.mode = "error"
+    val error = intercept[Error](qe.toString)
+    assert(error.getMessage.contains("error"))
+  }
+}