Refactor warnings and errors (#1728)

* Remove `err` and `warn`, keep `errx` and `warnx`, using them in RGBGFX too

* Separate RGBGFX and RGBLINK warnings/errors from main options

* Separate `report` function into `error` and `fatal` messages

* Implicit newlines for most RGBASM errors

src/link/assign.cpp

@@ -19,6 +19,7 @@
 #include "link/output.hpp"
 #include "link/section.hpp"
 #include "link/symbol.hpp"
+#include "link/warning.hpp"
 
 struct MemoryLocation {
 	uint16_t address;