Improve error handling for file writing

Fix; also removes error suppression from fclose().

In this improved version, export functions always return something, making error handling easier.

Most of those functions don't return anything right now, so this doesn't complete the project of nice error handling. It should avoid multiple error message for one problem. For example, if the fopen() fails, you get an error message for that. Then you get errors because the FALSE returned from fopen() is being used as if it was a file handle. With the patch, that won't happen.

If you are concerned about what happens when it returns false, it could return nothing. This would have the effect of having each error cause only one message instead of multiple.

That would be more elaborate. The current patch is supposed to just prevent multiple errors for one problem by stopping it from writing to non-existent file handles. It also removes some error suppression so that errors can be seen and fixed.

Here is a simpler patch which just avoids the duplicate error messages. Currently if a file can't be opened, you get the error for that plus the error when it tries to write to the non-existent file handle. With this patch, you get the first error then the function returns.