add error class, rm strange raise code

Maybe I'm missing something but 
`raise "command failed: #{invoke}" if ` looks like it is there by mistake so this pull is mainly to remove it.

Also, on Debian and Ubuntu I'm also finding with wkhtmltopdf 0.12.0 final (default installed by apt-get) a spec is failing:
 
```
/spec/pdfkit_spec.rb:337 
PDFKit#to_pdf should generate PDF if there are missing assets
     Failure/Error: pdf = pdfkit.to_pdf
     PDFKit::ImproperWkhtmltopdfExitStatus:
       Command failed (exitstatus=1): /usr/bin/wkhtmltopdf --disable-smart-shrinking --quiet --page-size Letter --margin-top 0.75in --margin-right 0.75in --margin-bottom 0.75in --margin-left 0.75in --encoding UTF-8 --ignore-load-errors - -
```
but don't have time to look at that now, thanks.