If a template file is missing the site will continue to display what it can. If it is a major template file (or a bunch of files) then the whole site may be unrecognizable or a blank page.
An error message could be added to the log file when a template file could not be found.
Geeklog looks in a number of spots for the file so we may only be able to report the last location check.
I am not sure how far we want to go in reporting this though... If a large number of template files are missing the error log will fill up quickly...
Here are a few ideas on how to handle this:
1) Maybe we should add a debug mode for a theme (set in the functions.php file) that if set to true will report any files missing.
2) Maybe like the proposed demo mode which would be set in the siteconfig.php we have a developer mode as will which can be set to display more advanced reporting.
One of the Geeklog Core Developers.