Mark exceeded stack frame scope error as Warning

[rjmholt]

Following on from @SeeminglyScience's explanation in PowerShell/PowerShellEditorServices#701 (comment), we tolerate a module-scope debugger stack frame error in an expected way.

We should do more to handle that stack frame error, but we should also mark it as WARNING rather than ERROR so that anyone reading the logs does not mistake it for a failing error.