B1G Harvest: handle 404s

[ewlarson]

Bad local image URL results in 404. Let's not try to process that request.body with MiniMagick

[d08ea5e1-9c1d-4d3a-a451-3004f939e647] [remote_content_type] text/html; charset=iso-8859-1
[d08ea5e1-9c1d-4d3a-a451-3004f939e647] [viewer_protocol] map
[d08ea5e1-9c1d-4d3a-a451-3004f939e647] [image_url] https://archive.lib.msu.edu/maps/public/images/494c-d_ae55e6v2_th
[d08ea5e1-9c1d-4d3a-a451-3004f939e647] [gblsi_thumbnail_uri] https://archive.lib.msu.edu/maps/public/images/494c-d_ae55e6v2_th
[d08ea5e1-9c1d-4d3a-a451-3004f939e647] [service_url] 
[d08ea5e1-9c1d-4d3a-a451-3004f939e647] [image_extension] .png
[d08ea5e1-9c1d-4d3a-a451-3004f939e647] [exception] #<ActiveRecord::RecordInvalid: Validation failed: Image Failed to manipulate with MiniMagick, maybe it is not an image? Original Error: `identify /var/folders/ys/2zqn1n7n44z0j7v_8v504fm1v4byym/T/mini_magick20180327-54497-nd8stc.png` failed with error:
identify: improper image header `/var/folders/ys/2zqn1n7n44z0j7v_8v504fm1v4byym/T/mini_magick20180327-54497-nd8stc.png' @ error/png.c/ReadPNGImage/4214.
>