Expand arti-client docs to include error reporting section

Quick MR to add a section in arti-client docs to let the developer know how best to handle errors, ie, we should encourage the use of the tor_error::Report trait via the .report() method rather than let the developer fend for themselves and become dismayed at the backtrace and seemingly undecipherable raw error output that comes from doing an unwrap() or even the raw Display output.

Note: this may need some more work on the language, since I'd initially written "error handling" everywhere but that is a misnomer.