Changing behavior when filter is not found

Hi,

When a filter is not installed (take here the example of `source-highlight`), asciidoc doesn't notice it, just throws a warning and don't render the document as if the filter was not needed.

The error is, in our case:
```
$ ./asciidoc.py /tmp/toto
/bin/sh: 1: source-highlight: not found
asciidoc: WARNING: toto: line 13: filter non-zero exit code: source-highlight -f xhtml -s shell: returned 127
asciidoc: WARNING: toto: line 13: no output from filter: source-highlight -f xhtml -s shell
```

What I propose is to have the `filter not found: %s` warning message if filter is not found and that the result of the filter_lines would be the "lines" args, so that, in our case, that would show the following warning, and the block of code will still be written in the bloc of code.

```
$ ./asciidoc.py /tmp/toto
asciidoc: WARNING: toto: line 13: filter not found: source-highlight
```
Joseph