Better error message when coalesce can't be resolved.

Author: marmbrus

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

@@ -37,7 +37,9 @@ case class Coalesce(children: Seq[Expression]) extends Expression {
   def dataType = if (resolved) {
     children.head.dataType
   } else {
-    throw new UnresolvedException(this, "Coalesce cannot have children of different types.")
+    val childTypes = children.map(c => s"$c: ${c.dataType}").mkString(", ")
+    throw new UnresolvedException(
+      this, "Coalesce cannot have children of different types. $childTypes")
   }
 
   override def eval(input: Row): Any = {