Correct error introduced in r23144

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/geps/gep-018-evidence-style@23194 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
Improve/harmonise error handling for missing template (even though missing template should never occur).

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/branches/geps/gep-018-evidence-style@23195 4ae1f11a-8b86-4847-b8af-ab372f36d1fd