Revise default $g_display_errors setting

The default now reflects the recommended Production settings, and a
better recommendation for developers is documented in the phpDoc
comment block.
It's there actually, [just a few lines above](https://github.com/mantisbt/mantisbt/pull/215/files#diff-dccc8d20b0765881557a770f05f6acb0R4081), as well as [in docbook](https://github.com/mantisbt/mantisbt/pull/215/files#diff-7d768fffb8c99682bfab61cb2ea39232R3503):

```
- DISPLAY_ERROR_NONE Suppress the error (no display). This is the default 
                     behavior for unspecified errors constants.
```

If you think it's not explained clearly enough, your suggestions for improvement are welcome.