[Security] Raise exception on IOException on StreamRepositoryService

The old code can be a security issue because the JasperReport will try
to get the recourse by other way when we return null.

core/src/main/java/org/mapfish/print/output/MapfishPrintRepositoryService.java

@@ -46,7 +46,7 @@ public InputStream getInputStream(final String uriString) {
           this.httpRequestFactory.createRequest(uri, HttpMethod.GET).execute();
       return new ResponseClosingStream(response);
     } catch (IOException e) {
-      return null;
+      throw new RuntimeException(String.format("Error on getting resource %s", uriString), e);
     }
   }