filter-not-found warning reports the executables it tried (not "None")

Since the exeutable is tatutologically not found, the filter-not-found warning alwasy says 'None': `class CasperJsStdoutFilter is not available because None not found`.

This changes it to call `executables` not `executable`: `class CasperJsSvg2PdfFilter is not available because [casperjs] not found`