Updates to error messages, canvas size calculation, documentation, examples

[patitsas]

I've added two new error messages: if the sun radius was not calculated, and if the sun radius is less than 50 pixels. This will handle Issue #4 . I also tweaked wording for fix_angle because it had confused me.

I updated the canvas size calculation to fix Issue #3 .

The link to example files on the README is broken, so I made three example sets for people to confirm the code is indeed working for them.

I updated README to reflect the addition of example cases. I removed the broken link. I also address the unstated assumption that sun radius is >50 pixels, gave guidance for if the code fails to compute a sun radius, and guidance for if the pip requirements step breaks.