[SPARK-16850][SQL] Improve type checking error message for greatest/least

## What changes were proposed in this pull request?
Greatest/least function does not have the most friendly error message for data types. This patch improves the error message to not show the Seq type, and use more human readable data types.

Before:
```
org.apache.spark.sql.AnalysisException: cannot resolve 'greatest(CAST(1.0 AS DECIMAL(2,1)), "1.0")' due to data type mismatch: The expressions should all have the same type, got GREATEST (ArrayBuffer(DecimalType(2,1), StringType)).; line 1 pos 7
```

After:
```
org.apache.spark.sql.AnalysisException: cannot resolve 'greatest(CAST(1.0 AS DECIMAL(2,1)), "1.0")' due to data type mismatch: The expressions should all have the same type, got GREATEST(decimal(2,1), string).; line 1 pos 7
```

## How was this patch tested?
Manually verified the output and also added unit tests to ConditionalExpressionSuite.

Author: petermaxlee <petermaxlee@gmail.com>

Closes #14453 from petermaxlee/SPARK-16850.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/conditionalExpressions.scala

@@ -299,7 +299,7 @@ case class Least(children: Seq[Expression]) extends Expression {
     } else if (children.map(_.dataType).distinct.count(_ != NullType) > 1) {
       TypeCheckResult.TypeCheckFailure(
         s"The expressions should all have the same type," +
-          s" got LEAST (${children.map(_.dataType)}).")
+          s" got LEAST(${children.map(_.dataType.simpleString).mkString(", ")}).")
     } else {
       TypeUtils.checkForOrderingExpr(dataType, "function " + prettyName)
     }
@@ -359,7 +359,7 @@ case class Greatest(children: Seq[Expression]) extends Expression {
     } else if (children.map(_.dataType).distinct.count(_ != NullType) > 1) {
       TypeCheckResult.TypeCheckFailure(
         s"The expressions should all have the same type," +
-          s" got GREATEST (${children.map(_.dataType)}).")
+          s" got GREATEST(${children.map(_.dataType.simpleString).mkString(", ")}).")
     } else {
       TypeUtils.checkForOrderingExpr(dataType, "function " + prettyName)
     }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/ConditionalExpressionSuite.scala

@@ -21,6 +21,7 @@ import java.sql.{Date, Timestamp}
 
 import org.apache.spark.SparkFunSuite
 import org.apache.spark.sql.catalyst.InternalRow
+import org.apache.spark.sql.catalyst.analysis.TypeCheckResult.TypeCheckFailure
 import org.apache.spark.sql.catalyst.dsl.expressions._
 import org.apache.spark.sql.types._
 
@@ -181,6 +182,12 @@ class ConditionalExpressionSuite extends SparkFunSuite with ExpressionEvalHelper
         Literal(Timestamp.valueOf("2015-07-01 10:00:00")))),
       Timestamp.valueOf("2015-07-01 08:00:00"), InternalRow.empty)
 
+    // Type checking error
+    assert(
+      Least(Seq(Literal(1), Literal("1"))).checkInputDataTypes() ==
+        TypeCheckFailure("The expressions should all have the same type, " +
+          "got LEAST(int, string)."))
+
     DataTypeTestUtils.ordered.foreach { dt =>
       checkConsistencyBetweenInterpretedAndCodegen(Least, dt, 2)
     }
@@ -227,6 +234,12 @@ class ConditionalExpressionSuite extends SparkFunSuite with ExpressionEvalHelper
         Literal(Timestamp.valueOf("2015-07-01 10:00:00")))),
       Timestamp.valueOf("2015-07-01 10:00:00"), InternalRow.empty)
 
+    // Type checking error
+    assert(
+      Greatest(Seq(Literal(1), Literal("1"))).checkInputDataTypes() ==
+        TypeCheckFailure("The expressions should all have the same type, " +
+          "got GREATEST(int, string)."))
+
     DataTypeTestUtils.ordered.foreach { dt =>
       checkConsistencyBetweenInterpretedAndCodegen(Greatest, dt, 2)
     }