topdown: Fix confusing error message for minus operator

Fixes open-policy-agent#1643

Signed-off-by: Torin Sandall <torinsandall@gmail.com>

test/cases/testdata/arithmetic/test-arithmetic-minus-type-error.yaml

@@ -0,0 +1,27 @@
+cases:
+- data:
+  modules:
+  - |
+    package test
+
+    p {
+      {1} - 1
+    }
+  note: arithmetic/minus/type error
+  query: data.test.p = x
+  want_error: operand 2 must be set but got number
+  want_error_code: eval_type_error
+  strict_error: true
+- data:
+  modules:
+  - |
+    package test
+
+    p {
+      1 - {1}
+    }
+  note: arithmetic/minus/type error
+  query: data.test.p = x
+  want_error: operand 2 must be number but got set
+  want_error_code: eval_type_error
+  strict_error: true

topdown/arithmetic.go

@@ -137,7 +137,11 @@ func builtinMinus(a, b ast.Value) (ast.Value, error) {
 		return nil, builtins.NewOperandTypeErr(1, a, "number", "set")
 	}
 
-	return nil, builtins.NewOperandTypeErr(2, b, "number", "set")
+	if ok2 {
+		return nil, builtins.NewOperandTypeErr(2, b, "set")
+	}
+
+	return nil, builtins.NewOperandTypeErr(2, b, "number")
 }
 
 func builtinRem(a, b ast.Value) (ast.Value, error) {