[VS Addin] Fix error handling on webserver startup error

We were previously clearing the output page after the error
message was printed meaning that the error was not showing
up in Visual Studio.  This error occurs if python.exe is not
found in PATH.

TEST=local testing

Review URL: https://codereview.chromium.org/955443003

git-svn-id: https://nativeclient-sdk.googlecode.com/svn/trunk/src@1580 050acbb0-2703-11df-ab0a-9f3f633ae91d
1 file changed