Go to RPCErrorCodes.ts and add the new code and message to the RPCErrorCodes object. The number used must be less than -32001 (it should just be the next number down from the last error code, serially). The message should be unique.
Go to errors.ts and add a new named class that extends the RPCError, passing the new error code you just created. For example:
exportclassTheErrorYouJustCreatedextendsRPCError{constructor({ ...args}={}){super({code: '-code you just created', ...args})}}
Go to handleErr function, and make a case for your error to be handled.