getting_started/4: 4.3: rewrite & reflow bits and pieces.¹

1: I got carried away, but tried to avoid changing the meaning of anything.
This is an area that still needs some work, but I'm not going to dig my boots
in.
Merge pull request #144 from komiga/gs-4-exception-rewrite

Rewrite & reflow majority of Getting Started section 4.3