Improve errors for cells

This set of changes resolve #3945 I think.  I've added a new framework for missing cell views as the MissingViewException references a controller which is total lies.

This also fixes an issue where relative paths would be inflected incorrectly, and solves broken layouts when missing cell/element exceptions are raised mid page.
PHP manual says _"This function discards the contents of the topmost output buffer and turns off this output buffering."_  So sounds like it only destroys topmost buffer.