Fix debug stepping problems when debugger is invoked from R RPC calls

A recent change for error management broke this scenario. We don't want to see error management functions on the callstack, so we hide them (and in fact all `addRpcHandler` functions). We also don't update line numbers in this case since we don't want the editor to show the RStudio error handler, which is technically the function on top of the stack. 

The fix is just to avoid treating user functions as hidden when they are invoked *by* a debugger internal function.