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).

help/errcount

@@ -36,7 +36,7 @@ EXAMPLE
 	    0
     ; a = 1/0; b = 2 + ""; c = error(27); d = newerror("a");
     ; print errcount(), a, errcount(), errmax();
-    4 Error 10001 4 10
+    4 Error E_DIVBYZERO 4 10
 
 LIMITS
     0 <= num < 2^32