Fix callback return values and clarify docs

[rcurtin]

This is a sibling PR to #382 and part of fixing #377.

I went through and audited the code to find places where the return values of callback functions were being ignored. I modified optimizers as necessary to handle those return values, attempting to terminate optimization before any more steps were taken.

I also modified the documentation to clarify that bools should be returned. However, for BeginOptimization() and EndOptimization() returning a bool doesn't make sense---so I made those return void.

In a couple places there were various other cleanups that I did, but nothing that affected functionality in any real way unrelated to callbacks.

Once this is done/merged, I also plan to update the technical report's section on callbacks and re-upload to arXiv.

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

[rcurtin]

I submitted an update to the arXiv technical report for ensmallen that clarifies the new callback structure.