prepare formatted content outside of write block

This is specifically to avoid having an exception
raised while the file is open for writing. Otherwise,
we end up with an empty file if something goes wrong.

pipenv/project.py

@@ -123,8 +123,10 @@ class Project(object):
     def write_toml(self, data, path=None):
         if path is None:
             path = self.pipfile_location
+
+        formatted_data = format_toml(toml.dumps(data))
         with open(path, 'w') as f:
-            f.write(format_toml(toml.dumps(data)))
+            f.write(formatted_data)
 
     @property
     def sources(self):