change error to warn and change test for it

vlib/v/checker/checker.v

@@ -2222,7 +2222,7 @@ fn (mut c Checker) block(mut node ast.Block) {
 		if node.stmts.len > 0 && node.stmts.last() is ast.ExprStmt {
 			last_stmt := node.stmts.last() as ast.ExprStmt
 			if last_stmt.expr !in [ast.CallExpr, ast.IfExpr, ast.MatchExpr, ast.InfixExpr] {
-				c.error('expression evaluated but not used', node.stmts.last().pos)
+				c.warn('expression evaluated but not used', node.stmts.last().pos)
 			}
 		}
 		c.stmts(mut node.stmts)

vlib/v/checker/tests/expr_evaluated_but_not_used.out

@@ -1,6 +1,7 @@
-vlib/v/checker/tests/expr_evaluated_but_not_used.vv:2:3: error: expression evaluated but not used
-    1 | fn main() {
-    2 |     {1, 2, 3, 4}
-      |      ^
-    3 |     println('works')
-    4 | }
+vlib/v/checker/tests/expr_evaluated_but_not_used.vv:7:3: warning: expression evaluated but not used
+    5 |         println(3)
+    6 |         println(4)
+    7 |         5
+      |         ^
+    8 |     }
+    9 |     println('works')

vlib/v/checker/tests/expr_evaluated_but_not_used.vv

@@ -1,4 +1,10 @@
 fn main() {
-	{1, 2, 3, 4}
+	{
+		println(1)
+		println(2)
+		println(3)
+		println(4)
+		5
+	}
 	println('works')
 }