Fail on url(directory)

[NielsHolt]

If the url(..) is a directory the task fails
Some packages using artificial contents in css-classes to pass info to JavaScript
Eg. leaflet using
.leaflet-default-icon-path {
background-image: url(images/);
}
Proposal:
Try using grunt.file.isFile instead of grunt.file.exists to test for file existing

[mihhail-lapushkin]

Fixed in 1.7.0.