Minor improvements after testing with some "noisy" data

This repo worked extremely well with a bunch of test emf images I imported using it (>10K) - because this data was a bit noisy`*` I made a few very minor improvements just to handle this noise/bad inputs: 

Although it looks like a lot of changes, it is really just a few:

1. Added some checks that input image files exist
2. Added an exception handler to the row processing (this is the one that made it look like more changes than reality)
3. Added the ability to use tag other than "vector" for these (I needed to use tag "svg")
4. Added a few switches to turn off a few things I personally didn't need (e.g. serverstyle, but kept the defaults as they were originally)

Feel free to use/merge/ignore at your leisure - just wanted to give back some fixes, since your repo really, really helped me - thanks again for making it public.

`*` Just in case interested, large set of source data used was here: https://github.com/Esri/joint-military-symbology-xml/blob/master/samples/imagefile_name_category_tags