Inconsistent error handling in invalid mustache templates

When rendering mustache templates containing unexistent/undisplayable variables, error handling is inconsistent.

When running the attached file, the file promises fails with a

Error rendering mustache template '(null)'

error, while the string_mustache is not executed at all, with no error message (even in debug).

It should at least display an error (as we don"t have proper error handling for vars promises).