Throw a more interpretable error when old CLIs encounter new Core ver…

…sions (#1607)

core/index.ts

@@ -17,7 +17,15 @@ const session = globalSession();
 
 const supportedFeatures = [dataform.SupportedFeatures.ARRAY_BUFFER_IPC];
 
-// These exports constitute the public API of @dataform/core. 
+// Older versions of the CLI are not compatible with Core version ^3.0.0, and throw when this method
+// is not available. Instead this more interpretable error message is thrown.
+// Note: for future backwards compatability breaking changes, the exported "version" variable should
+// be used instead.
+function indexFileGenerator() {
+  throw new Error("@dataform/cli ^3.0.0 required.");
+}
+
+// These exports constitute the public API of @dataform/core.
 // They must also be listed in packages/@dataform/core/index.ts.
-// Changes to these will break @dataform/api, so take care!
-export { compiler, main, session, supportedFeatures, version };
+// Changes to these will break @dataform/cli, so take care!
+export { compiler, indexFileGenerator, main, session, supportedFeatures, version };

packages/@dataform/core/index.ts

@@ -1 +1 @@
-export { compiler, main, session, supportedFeatures, version } from "df/core";
+export { compiler, indexFileGenerator, main, session, supportedFeatures, version } from "df/core";