Super user invites (#483)

This PR brings user invites logic to the Management service
via HTTP API. 
The POST /users/ API endpoint creates a new user in the Idp
and then in the local storage. 
Once the invited user signs ups, the account invitation is redeemed.
There are a few limitations.
This works only with an enabled IdP manager.
Users that already have a registered account can't be invited.

management/server/error.go

@@ -0,0 +1,52 @@
+package server
+
+import (
+	"fmt"
+)
+
+const (
+	// UserAlreadyExists indicates that user already exists
+	UserAlreadyExists ErrorType = 1
+	// AccountNotFound indicates that specified account hasn't been found
+	AccountNotFound ErrorType = iota
+	// PreconditionFailed indicates that some pre-condition for the operation hasn't been fulfilled
+	PreconditionFailed ErrorType = iota
+)
+
+// ErrorType is a type of the Error
+type ErrorType int32
+
+// Error is an internal error
+type Error struct {
+	errorType ErrorType
+	message   string
+}
+
+// Type returns the Type of the error
+func (e *Error) Type() ErrorType {
+	return e.errorType
+}
+
+// Error is an error string
+func (e *Error) Error() string {
+	return e.message
+}
+
+// Errorf returns Error(errorType, fmt.Sprintf(format, a...)).
+func Errorf(errorType ErrorType, format string, a ...interface{}) error {
+	return &Error{
+		errorType: errorType,
+		message:   fmt.Sprintf(format, a...),
+	}
+}
+
+// FromError returns Error, true if the provided error is of type of Error. nil, false otherwise
+func FromError(err error) (s *Error, ok bool) {
+	if err == nil {
+		return nil, true
+	}
+	if e, ok := err.(*Error); ok {
+		return e, true
+	}
+	return nil, false
+}