Merge pull request #57 from rastapasta/create-folder-error

Throw an error if creating a folder fails.

src/TileSource.js

@@ -168,7 +168,8 @@ class TileSource {
       fs.mkdirSync(path);
       return true;
     } catch (error) {
-      return error.code === 'EEXIST';
+      if (error.code === 'EEXIST') return true;
+      throw error;
     }
   }
 }