Throw error in load_map if static "file" attribute is used but file does not exist

[springmeyer]

..and if the file does not contain [] indicating runtime evaluation of the PathExpression.

The reason this is needed is that otherwise Mapnik prints N warnings of the file not being found for N features. Ensuring files exist at XML load is reasonable and a good way to mitigate peoples logfiles from bloating to the same size as their data the second that a map is rendered which is missing symbol files just due to a typo or permissions error.