Add more context aware error for policy parsing errors (#8726)

In existing functionality we simply return a generic
error such as "MalformedPolicy" which indicates just
a generic string "invalid resource" which is not very
meaningful when there might be multiple types of errors
during policy parsing. This PR ensures that we send
these errors back to client to indicate the actual
error, brings in two concrete types such as

 - iampolicy.Error
 - policy.Error

Refer #8202

pkg/iam/policy/error.go

@@ -0,0 +1,39 @@
+/*
+ * MinIO Cloud Storage, (C) 2019 MinIO, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package iampolicy
+
+import "fmt"
+
+// Error generic iam policy error type
+type Error struct {
+	Err string
+}
+
+// Errorf - formats according to a format specifier and returns
+// the string as a value that satisfies error of type iampolicy.Error
+func Errorf(format string, a ...interface{}) error {
+	return Error{Err: fmt.Sprintf(format, a...)}
+}
+
+// New initializes a new Error
+func New(err string) error {
+	return Error{Err: err}
+}
+
+func (e Error) Error() string {
+	return e.Err
+}