change error printing from errnum to errsym

When printing an error, calc used to print the errnum (error number):

    ; 1/0
	    Error 10001

Calc now prints the errsym (errsym):

    ; 1/0
	    Error E_DIVBYZERO

Added errsym E_LN_3 for ln(0).
Added errsym E_LOG_5 for log(0).
Added errsym E_LOG2_4 for log2(0).
Added errsym E_LOGN_6 for logn(0,base).

symbol.c

@@ -241,10 +241,20 @@ printtype(VALUE *vp)
 {
 	int	type;
 	char	*s;
+	char *errsym;
+	bool alloced;
 
 	type = vp->v_type;
 	if (type < 0) {
-		printf("Error %d", -type);
+		errsym = errnum_2_errsym(-type, &alloced);
+		if (errsym == NULL) {
+			printf("Error %d", -type);
+		} else {
+			printf("Error %s", errsym);
+			if (alloced == true) {
+				free(errsym);
+			}
+		}
 		return;
 	}
 	switch (type) {