Improve "const" error message #1079.

test/test_suite/constants/assign_to_const.c3

@@ -0,0 +1,8 @@
+module test;
+import std::io;
+
+fn void main()
+{
+	const NUM = 4;
+	NUM += 1; // #error: You cannot assign to a constant expression
+}

test/test_suite/expressions/arithmetics_sema_fail.c3

@@ -17,32 +17,32 @@ fn void test9()
 
 fn void test10()
 {
-    10 = 20;        // #error: An assignable expression
+    10 = 20;        // #error: to a constant expression
 }
 
 fn void test11()
 {
-    '10' = '20';        // #error: An assignable expression
+    '10' = '20';        // #error: to a constant expression
 }
 
 fn void test12()
 {
-    true = false;        // #error: An assignable expression
+    true = false;        // #error: to a constant expression
 }
 
 fn void test13()
 {
-    "a" = "b";        // #error: An assignable expression
+    "a" = "b";        // #error: to a constant expression
 }
 
 fn void test14()
 {
-    1.2 = 1.3;        // #error: An assignable expression
+    1.2 = 1.3;        // #error: to a constant expression
 }
 
 fn void test15()
 {
-    null = null;      // #error: An assignable expression
+    null = null;      // #error: to a constant expression
 }
 
 fn void test16()

test/test_suite/expressions/assignability.c3

@@ -2,27 +2,27 @@ def Number = int;
 
 fn void test1()
 {
-    10 = 20;        // #error: An assignable expression
+    10 = 20;        // #error: to a constant expression
 }
 
 fn void test2()
 {
-    "foo" = "bar";  // #error: An assignable expression
+    "foo" = "bar";  // #error: to a constant expression
 }
 
 fn void test3()
 {
-    true = false;   // #error: An assignable expression
+    true = false;   // #error: to a constant expression
 }
 
 fn void test4()
 {
-    'c' = 'd';      // #error: An assignable expression
+    'c' = 'd';      // #error: to a constant expression
 }
 
 fn void test5()
 {
-    3.14 = 2.14;    // #error: An assignable expression
+    3.14 = 2.14;    // #error: to a constant expression
 }
 
 fn void test21()