Add Unwrap() method to standard error types

Address #70

error.go

@@ -63,6 +63,10 @@ func (e *ConnectionError) Timeout() bool {
 	return false
 }
 
+func (e *ConnectionError) Unwrap() error {
+	return e.error
+}
+
 // ContextError is returned when request has been ended with context timeout or cancel.
 type ContextError struct {
 	error
@@ -88,6 +92,10 @@ func (e *ContextError) Timeout() bool {
 	return e.CtxErr == context.DeadlineExceeded
 }
 
+func (e *ContextError) Unwrap() error {
+	return e.CtxErr
+}
+
 // QueryError is returned when query error has been happened.
 // It has error Code.
 type QueryError struct {
@@ -113,6 +121,10 @@ func (e *QueryError) Timeout() bool {
 	return false
 }
 
+func (e *QueryError) Unwrap() error {
+	return e.error
+}
+
 // UnexpectedReplicaSetUUIDError is returned when ReplicaSetUUID set in Options.ReplicaSetUUID is not equal to ReplicaSetUUID
 // received during Join or JoinWithSnap. It is only an AnonSlave error!
 type UnexpectedReplicaSetUUIDError struct {
@@ -136,6 +148,10 @@ func (e *UnexpectedReplicaSetUUIDError) Is(target error) bool {
 	return ok
 }
 
+func (e *UnexpectedReplicaSetUUIDError) Unwrap() error {
+	return e.QueryError
+}
+
 var _ Error = (*ConnectionError)(nil)
 var _ Error = (*QueryError)(nil)
 var _ Error = (*ContextError)(nil)