Add exception to prevent invalid configuration

[michael-misshore]

manifest and css path(s) cannot/should not be both defined.

Addresses #45

[keferboeck]

Doesn't solve the issue - just tried with this specific commit : ( - but still getting this:

Error: ENOENT: no such file or directory, open '/assets/application-fe612b1419b5a8d61dfc3dbc0fe5e5e58683baab1701e8e5870e475209314419.css'
Failed to get CSS for route /
  with options={"url"=>"http://localhost:3000/", "css"=>"/assets/application-fe612b1419b5a8d61dfc3dbc0fe5e5e58683baab1701e8e5870e475209314419.css", "width"=>1300, "height"=>900, "timeout"=>30000, "forceInclude"=>[], "strict"=>false, "maxEmbeddedBase64Length"=>1000, "userAgent"=>"Penthouse Critical Path CSS Generator", "renderWaitTime"=>100, "blockJSRequests"=>true, "customPageHeaders"=>{"Accept-Encoding"=>"identity"}}

[kevinhq]

Doesn't solve the issue - just tried with this specific commit : ( - but still getting this:

Error: ENOENT: no such file or directory, open '/assets/application-fe612b1419b5a8d61dfc3dbc0fe5e5e58683baab1701e8e5870e475209314419.css'
Failed to get CSS for route /
  with options={"url"=>"http://localhost:3000/", "css"=>"/assets/application-fe612b1419b5a8d61dfc3dbc0fe5e5e58683baab1701e8e5870e475209314419.css", "width"=>1300, "height"=>900, "timeout"=>30000, "forceInclude"=>[], "strict"=>false, "maxEmbeddedBase64Length"=>1000, "userAgent"=>"Penthouse Critical Path CSS Generator", "renderWaitTime"=>100, "blockJSRequests"=>true, "customPageHeaders"=>{"Accept-Encoding"=>"identity"}}

Same here. This is merged to 4.0.1, right?