[olympic] Add error handling back to developer-handbook.md

developer-handbook.md

@@ -169,6 +169,14 @@ Olympic builds on Ox as its standard-ish library.
 Please read the [Ox documentation](deps/ox/ox-docs.md).
 The Ox way of doing things is the Olympic way of doing things.
 
+### Error Handling
+
+The GBA build has exceptions disabled.
+Instead of throwing exceptions, all engine code should return
+[ox::Errors](deps/ox/ox-docs.md#error-handling) for error reporting.
+For the sake of consistency, try to stick to ```ox::Error``` in non-engine code
+as well, but non-engine code is free to use exceptions when they make sense.
+
 ### File I/O
 
 All engine file I/O should go through Keel, which should go through