PR#631: cg_import fails immediately if build directory already exists

Merges #631
https://pagure.io/koji/pull-request/631

Merges #425
https://pagure.io//koji/issue/425

hub/kojihub.py

@@ -5225,6 +5225,7 @@ class CG_Importer(object):
 
         self.assert_policy()
         self.set_volume()
+        self.check_build_dir()
 
         koji.plugin.run_callbacks('preImport', type='cg', metadata=metadata,
                 directory=directory)
@@ -5312,6 +5313,13 @@ class CG_Importer(object):
             self.buildinfo['volume_name'] = vol['name']
 
 
+    def check_build_dir(self):
+        """Check that the import directory does not already exist"""
+        path = koji.pathinfo.build(self.buildinfo)
+        if os.path.lexists(path):
+            raise koji.GenericError("Destination directory already exists: %s" % path)
+
+
     def prep_build(self):
         metadata = self.metadata
         buildinfo = get_build(metadata['build'], strict=False)