Document new errhand

[bjornbytes]

• lovr.errhand isn't set by default now, and instead lives in a separate errhand.lua file included as part of the default nogame bundle, allowing it to be loaded on demand/customized/removed
• You can require it yourself if you want to wrap it
• The example with the default errhand should be updated (or maybe just linked to?)
• Mention that it is overwritten when fusing