Make exception messages user friendly.

Author: yhuai

sql/core/src/main/scala/org/apache/spark/sql/sources/ddl.scala

@@ -198,7 +198,7 @@ object ResolvedDataSource {
               .asInstanceOf[org.apache.spark.sql.sources.SchemaRelationProvider]
               .createRelation(sqlContext, new CaseInsensitiveMap(options), schema)
           case _ =>
-            sys.error(s"${clazz.getCanonicalName} should extend SchemaRelationProvider.")
+            sys.error(s"${clazz.getCanonicalName} does not allow user-specified schemas.")
         }
       }
       case None => {
@@ -208,7 +208,7 @@ object ResolvedDataSource {
               .asInstanceOf[org.apache.spark.sql.sources.RelationProvider]
               .createRelation(sqlContext, new CaseInsensitiveMap(options))
           case _ =>
-            sys.error(s"${clazz.getCanonicalName} should extend RelationProvider.")
+            sys.error(s"A schema needs to be specified when using ${clazz.getCanonicalName}.")
         }
       }
     }

sql/core/src/test/scala/org/apache/spark/sql/sources/TableScanSuite.scala

@@ -314,4 +314,34 @@ class TableScanSuite extends DataSourceTest {
       sql("SELECT * FROM oneToTenDef"),
       (1 to 10).map(Row(_)).toSeq)
   }
+
+  test("exceptions") {
+    // Make sure we do throw correct exception when users use a relation provider that
+    // only implements the RelationProvier or the SchemaRelationProvider.
+    val schemaNotAllowed = intercept[Exception] {
+      sql(
+        """
+          |CREATE TEMPORARY TABLE relationProvierWithSchema (i int)
+          |USING org.apache.spark.sql.sources.SimpleScanSource
+          |OPTIONS (
+          |  From '1',
+          |  To '10'
+          |)
+        """.stripMargin)
+    }
+    assert(schemaNotAllowed.getMessage.contains("does not allow user-specified schemas"))
+
+    val schemaNeeded = intercept[Exception] {
+      sql(
+        """
+          |CREATE TEMPORARY TABLE schemaRelationProvierWithoutSchema
+          |USING org.apache.spark.sql.sources.AllDataTypesScanSource
+          |OPTIONS (
+          |  From '1',
+          |  To '10'
+          |)
+        """.stripMargin)
+    }
+    assert(schemaNeeded.getMessage.contains("A schema needs to be specified when using"))
+  }
 }