implement reporting BASIC errors, continuing work on tokeniser

c64/basic/do_basic_error.s

@@ -0,0 +1,31 @@
+	;; Error # in X
+
+do_basic_error:
+	;; "?"
+	txa
+	pha
+	lda #$3f
+	jsr $ffd2
+	pla
+	tax
+
+	;; Error message text
+	jsr print_packed_message
+
+	lda #$20
+	jsr $ffd2
+	
+	;; "ERROR"
+	ldx #33
+	jsr print_packed_message
+
+	;; New lines
+	lda #$0d
+	jsr $ffd2
+	lda #$00
+	jsr $ffd2
+
+	;; XXX - Restore stack depth first?
+	
+	jmp basic_main_loop
+