Better spanned error messaging

[ariasanovsky]

I would love to take more advantage of span.
Most of the errors currently point to the macro instead of the problematic span.
I am open to suggestions!
If you would like to improve this, please either:

1. add an appropriate tests.rs to demo the error
2. expand examples/ to demo the improvement (e.g., "uncomment this line to see the beautiful error")

[ariasanovsky]

not specific, and redundant w.r.t. issue #4 merge