frontened: lexer: Print error on bad token

Signed-off-by: Ian Moffett <ian@mirocom.org>

frontend/lexer.c

@@ -18,6 +18,7 @@
 #include "frontend/token.h"
 #include "frontend/lexer.h"
 #include "common/ptrbox.h"
+#include "common/trace.h"
 
 /*
  * Test if a given character counts as a whitespace character
@@ -232,6 +233,7 @@ lexer_scan(struct quip_state *state, struct token *tokres)
             return 0;
         }
 
+        trace_error(state, "unknown token %c\n", c);
         break;
     }