Improve error message for non-existing files

If you refer to a non-existing file

:constdata:label:`non-existing.csv?foo`

the error message in build output is ERROR: Missing :constdata:label: template for 'non-existing.csv'.. It should be that the file is missing.

If the non-existing.csv has the template, but is still missing, error is ERROR: Cache DB error (exception: no such table: problematic.csv). It should be still that file is missing.