..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.
load in load_map if image/svg files are not available when not using …
…stock markers or dynamic expressions - closes #1439