Handling image file cell errors

[GrandyB]

Is your feature request related to a problem? Please describe.
Right now, in order for a sheet to be properly ready for production, you have to add in a bunch of guards around cells that are an image source.
This is to prevent errors being thrown in your face and files not being updated when an image source cell is empty/malformed/has unexpected content.

These guards are annoying to constantly have to add in; would rather not need them and be a bit more permissive, instead writing out a transparent image in more scenarios.

Roughly related to #38 which added this in some form, and #39 .

Describe the solution you'd like
I'm thinking of changing the handling around this to the following:

1. Empty image cell = transparent 50x50px image written, no error popup, debug log to say it has done that.
2. Cell has "#N/A" as content = same as above.
3. Cell with malformed URL or issue writing the image = error thrown in face.

Questions
Does the 50x50 need to be configurable? if so, application or config specific?
Would you rather have #N/A throw error in face?

[GrandyB]

Released in 0.7.3