Best design pattern for structured sequential handling

Doing maintenance on a project I came across code, which I find unnecessary hard to read and I wish to refactor, to improve readability.

The functionality is a long chain of actions that need to be performed sequentially. The next action should only be handled if the previous action was successful. If an action is not successful a corresponding message needs to be set. And the returned type is a Boolean. (successful true/false). Just like the return type of all the called actions.

Basically it comes down to something like this.

string m = String.Empty; // This is the (error)Message.
bool x = true; // By default the result is succesful.

x = a1();
if(x) {
    x = a2();
}
else {
    m = "message of failure a1";
    return x;
}

if(x) {
    x = a3();
}
else {
    m = "message of failure a2";
    return x;
}

//etcetera..etcetera...

if(x){
    m = "Success...";
}
else{
    m = "Failure...";
}

return x;

My question is: What is a better structure / pattern to handle this kind of logic?

Main goals are:

• increase readability.
• increase maintainability.

Please keep in mind that it is quite a large chain of actions that is being performed sequentially. (Thousands lines of code)