[Minor][SQL] Use same function to check path parameter in JSONRelation

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #4649 from viirya/use_checkpath and squashes the following commits:

0f9a1a1 [Liang-Chi Hsieh] Use same function to check path parameter.

sql/core/src/main/scala/org/apache/spark/sql/json/JSONRelation.scala

@@ -37,7 +37,7 @@ private[sql] class DefaultSource
   override def createRelation(
       sqlContext: SQLContext,
       parameters: Map[String, String]): BaseRelation = {
-    val path = parameters.getOrElse("path", sys.error("Option 'path' not specified"))
+    val path = checkPath(parameters)
     val samplingRatio = parameters.get("samplingRatio").map(_.toDouble).getOrElse(1.0)
 
     JSONRelation(path, samplingRatio, None)(sqlContext)
@@ -48,7 +48,7 @@ private[sql] class DefaultSource
       sqlContext: SQLContext,
       parameters: Map[String, String],
       schema: StructType): BaseRelation = {
-    val path = parameters.getOrElse("path", sys.error("Option 'path' not specified"))
+    val path = checkPath(parameters)
     val samplingRatio = parameters.get("samplingRatio").map(_.toDouble).getOrElse(1.0)
 
     JSONRelation(path, samplingRatio, Some(schema))(sqlContext)

sql/hive/src/test/scala/org/apache/spark/sql/hive/MetastoreDataSourcesSuite.scala

@@ -547,7 +547,7 @@ class MetastoreDataSourcesSuite extends QueryTest with BeforeAndAfterEach {
         Map.empty[String, String])
     }.getMessage
     assert(
-      message.contains("Option 'path' not specified"),
+      message.contains("'path' must be specified for json data."),
       "We should complain that path is not specified.")
 
     sql("DROP TABLE savedJsonTable")