Add warning for images without max-pixel

[yanokwa]

Closes #475

Why is this the best possible solution? Were any other approaches considered?

It's the narrowest change. I used the shortest possible warning to make it easier to read.

Note there is one unrelated commit to make the Learn/read more consistent throughout pyxform.

What are the regression risks?

None that I can think of. It's just a warning. If you have a lot of images, you could get a lot of these warnings. I toyed with the idea of de-duping it so you only get one generic one, but I think row numbers are more helpful.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

No.

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests_v1
• run nosetests and verified all tests pass
• run black pyxform to format code
• verified that any code or assets from external sources are properly credited in comments

[lognaturel]

Note that this could be annoying for folks who use client settings to configure this. I think in general we do want to steer folks towards making the form as specific with these kinds of requirements as possible. Tagging @danbjoseph in case he wants to chime in since I do remember he told me he used the setting instead of this.