Formatting invalid code causes entire file to be cleared.

[newtonallen3]

Repro steps:

1. Create an invalid go file
2. Format it (Shift+Ctrl+F)

Observed behavior:
Everything in the file is deleted. An empty file is left behind. The console window shows the output of gofmt, which evidently terminated with an error code and printed some messages to stderr.

Expected behavior:
The file is unmodified. Perhaps an error dialog appears, showing that editing failed.