Improve error message when a symbol is not constant

src/asm/symbol.c

@@ -269,7 +269,7 @@ uint32_t sym_GetConstantValue(char const *s)
 		if (sym_IsConstant(psym))
 			return getvaluefield(psym);
 
-		fatalerror("Expression must have a constant value");
+		fatalerror("\"%s\" does not have a constant value", s);
 	}
 
 	yyerror("'%s' not defined", s);

test/asm/bracketed-symbols.err

@@ -1,4 +1,4 @@
 ERROR: bracketed-symbols.asm(16):
     Print types are only allowed for numbers
 ERROR: bracketed-symbols.asm(20):
-    Expression must have a constant value
+    "Label" does not have a constant value